Physics the Why vs. How question? I've been told that a physicist never asks why and must always ask how. Is this true? Have all discoveries in physics been due to asking how? or have some been discovered by asking why?
 A: In English, asking 'why' is a request for an explanation of, reasons for, or purpose of something.
Physics does provide explanations and some types of reasons, but doesn't concern itself will questions of purpose at all, at least not in any teleological sense.
So physics answers some types of why questions but not others, because in English usage 'why' is overloaded and can be used to ask for many different kinds of things.
A: Here are my two cents of the euro:
Physics is a science that has a large body of observations, and  a limited number of mathematical models/theories that aim to organize and explain those observations and , very important, get validated by predicting the behavior of new observations.
Mathematical theories start with axioms and some tools that develop theorems from those axioms and then various setups can be examined. 
For example Euclidean geometry starts with axioms and ends with being able to predict and design complicated geometrical shapes.  One can start asking why the sum of angles in a triangle is 180 degrees and one can prove it using the tools. If one goes further up in the why questions one ends up with the axioms. If one changes the axioms then a different geometry is implied. So "why" questions end up on the axioms. One could as well ask "how" one gets 180degrees for the sum of the angles of the triange, and then the "why" goes to "why start with these axioms".
In physics the theories in addition to the mathematical construct has   equivalent to axioms, postulates. These have been postulated because of the need for the mathematical model to agree with measurements and data in general. For example the Heisenberg Uncertainty principle. Or the "square of the wavefunction gives the probability of the measurement". In a similar manner as in the above mathematical example, all the "why" questions in physics are really answered as "how" one goes from the axioms and postulates to the specific observational data or predictions. The "why" ends up on the axioms for the mathematics and postulates for physics. And the answer then is "because" these basic assumptions/postulates  are necessary to fit our mathematical  model to the existing data  and give us confidence in predictions for new observations. 
The only answer to "why" is this "because", data says so. 
Edition after comment by Ingo :
Once one has a theory, and physics is really as I stated in the first paragraph a mathematical theory that organizes known data in order to be able to predict future unmeasured ones, the "how" question gives the causal path in our understanding of how the final data/observations happened and how the predicted ones will appear.  Why questions address the existential state. When we have no theory and have an observation we start with "Why", because the observation exists. When a theory forms then it is the causal path that is sought and "why" goes up the mathematical ladder by "how mathematically this happens"   transferring the existential question to the axioms and postulates.
A: "Why" is the question for philosophers.  "How" is the question for physicists.
The task of a physical scientist is to find a model that adequately describes "how" nature behaves.  When more than one model fits, see Occam's Razor.
The question of "why" nature is described by one model or another is best left to the philosopher and, be aware, that path leads to madness.
A: "Why" is the fundamental scientific question, the question regarding cause and effect. "How" already implies that certain things cause others, and sometimes it seems that cause and effect are not that easy to distinguish. You can't do any science without "why".
Take, for example, the question: "How does CO2 contribute to climate change?" 
This is a scientific question only after it has been established that


*

*the climate changes

*it does this because of green house gases in the athmosphere

A: Physics is concerned with abstract, mathematical models that describe the behaviour of particles, space, time etc and that are tested with observations and experiments. If a model is in agreement with experiments, under certain conditions and up to a certain accuracy, then it is accepted (for situations where those conditions hold). But a theory remains an abstract model; it provides a way in which nature can be described, but doesn't say what nature is. So one should always be cautious about interpreting theories and taking those interpretations too far. 
Richard Feynman illustrated this in one of his lectures (his argument starts around 5:30 in the first clip and end around 7:30 in the second clip):
The Relation of Mathematics and Physics - Part 4
The Relation of Mathematics and Physics - Part 5
In the clips, Feynman shows that Newtonian gravity can be described in 3 different ways: action at a distance, a potential field, and the principle of least action. Conceptually, these 3 descriptions are very different. However, mathematically, they give exactly the same results. This means that, as far as physics is concerned, all three are equally valid and one has no reason to prefer one above the others. There is no unique "right" answer.
In fact, as Feynman argued, it is a bonus to have more than one description, because they enable us to look at new problems from multiple angles, and they offer us more routes and tools to find new theories.
A: anna v wrote in her answer:

The only answer to "why" is this "because", data says so.

I'd like to provide another example. Consider the question "How does matter move through space". In trying to answer this, Aristotelian arguments could satisfy someone of the answer. Prodding further, Aristotelian arguments don't hold up and we need to use Newtonian arguments. Even further, one could do a double slit experiment and show that, in fact, a Newtonian particle description is inadequate and a quantum mechanical description is needed. In each of these cases there already has to be a framework where we're satisfied with an answer. I agree that "$\dot{x}=\text{constant}$" could be a description of "how" the particle moves, but that's assuming we're working with a Newtonian viewpoint. If we don't allow ourselves to use a Newtonian viewpoint, then that's not how the particle moves at all!
So, if answers to "why" are, "because the data says so" or "because that's the idealization we're considering", how are answers to "how" different? (and why?!)
In the related philosophy.se post, ChristopherE writes:

Your other question is "Are 'why' questions" (broadly understood) useful in or applicable to the study of science?" Certainly, yes, and there needn't be anything revolutionary about them.

I don't think this "why" vs "how" business gets to the heart of the matter, and of course the linguistic nature of "why" and "how" obscures things further. Maybe it would be better rephrased as "physics doesn't prove things in the mathematical sense". Ask "why" "what" or "how", but don't suppose you get a leg up on nature just because the resulting models are predictive. 

disclaimer: I'm a baby physicist
A: What science tries to do is paint as complete and consistent of picture in the language of mathematics as possible.
This in practice means figuring out how pieces fit together rather than tackling questions like why is there a puzzle to begin with.
A: English is pretty imprecise and asking questions of the form "Why does ..." feels natural even when "How does ..." would suffice.  "Why" has philosophical overtones because it goes beyond just "How".  Physics is a science and the scientific method is tried-and-true method for gaining knowledge about how things work.
I'd suggest that in any place you have a question "Why does ..." try to substitute it with "How does ..." instead.  Any portion of "Why does ..." not answered by "How does ..." is the philosophical / non-scientific portion of the question.
For example "Why does 1 + 2 equal 3?" can be substituted with "How does 1 + 2 equal 3?" and answered adequately.  Depending on how deep and technical you want to get, the answer may delve into the definition of addition, various axioms, peano arithmetic, and so on.  If you still have a question about "why" after all of that, your question is of a philosophical nature.
Physics is firmly rooted in the scientific method, mathematics, and experiment.  It simply can't answer the philosophical portions of questions.
A: I agree with the others that physics is fundamentally a "hard science" which is (or should be) concerned exclusively with physical, empirical facts, but I have to disagree with the assertion that "How?" is a scientific question while "Why?" is not. Moreover, philosophy is central to scientific research despite not being itself the end goal. This discussion with all of its deep linguistic analyses bears witness to the importance if not the usefulness of not only understanding nature but also understanding how we understand nature.
Plenty of sound scientific questions have begun with "why." Faraday wondered why current deflects a compass needle, and why the galvanometer attached to a circuit jumped whenever he closed the switch on another circuit. Many scientists of the early twentieth century asked why beta decay appeared to violate otherwise sacrosanct conservation laws. Newton asked why Kepler's Laws were so accurate. None of these questions can be adequately rephrased as "how" questions because they all arise precisely from a deep and thorough understanding of "how," an understanding left frustrated by inadequate explanation.
For this reason, these sorts of questions - the questions of curiosity that challenge the limits of our theories and guess at what might lie beyond - are exactly those which allow science to grow and flourish. Remember: science is only as good as the people who practice it, and no theory is truth, every theory (no matter how accurate and well-established) is somebody's attempt to make reality more palatable.
"How" is necessary and important, don't get me wrong, but so is "why." As Thomas Kuhn might put it: "How" is status quo science, journeyman science, science "as usual"; "why" is fringe science, confrontational science, paradigm shift.
The truth is that it is disingenuous to sacrifice "why" on the altar of "how." Forcing our language into some tightly defined pen where only "how" is allowed is simply a false dichotomy born of a mistaken appreciation of natural language.
And just to satisfy all of the empiricists and positivists out there, allow me to present you with test case number 1 for "how" gone wrong: thermoelectric theory. Thermoelectric theory in the mid twentieth century was just plain wrong. It was built on a flawed phenomenological model that was not adequately challenged at the time, and as a result, it was not until pretty recently that anyone thought that it was even possible to produce a material with a Seebeck Coefficient greater than one. That model was built on "how." Like all phenomenological models, it did its best to fit experimental data simply and elegantly, and many people failed to miss the point that it fit experiments so well because it was made to do that. It wasn't predictive, but it was accurate in the only cases that were ever tested. No one complained about the "why" since the "how" seemed so right, but it wasn't.
Without "why" to challenge "how," we're nothing but theoretical engineers or practical mathematicians. Physics - true physics - is fathered by "how" but born of "why." Speaking for myself and many others, I love and study the how, but only because I am intoxicated by the pursuit of why. "How" may be the wheels on the road, but "why" is the engine of the car. We need them both to move forward.
Edit: After the useful conversations at philosophy.se and english.se, I feel that it's worth adding one more idea here. (Or perhaps just saying an old idea another way.)
"Why" questions challenge theories or speculate about possibilities. They allow us to address the most difficult issues of science which develop when we run up against the boundaries of our knowledge. Furthermore, they are useful when the problem at hand is characterized by hidden information or unknown parameters which are strongly affecting the experiment. For instance, in the beta decay example above, it was not possible to ask "How do neutrinos affect beta decay?" because the idea of neutrinos had not been invented yet; in fact, "Neutrinos." is precisely the answer to the question "Why does beta decay seem to violate conservation of momentum?"
But, of course, neutrinos could only be postulated because so many scientists asked the questions "How does beta decay behave under these conditions?" or "How does it behave under those conditions?" By accruing data (which are the answers to "how" questions), they were able to identify discrepancies between observation and expectation which required new theories and creative thinking to adequately explain.
"How" questions generally are more tightly focused and lend themselves to being phrased as implicit hypotheses which can be tested, but they necessarily live within the context of a theory and, therefore, presuppose some fore-knowledge. "Why" questions, on the other hand, generally strike at the heart of a scientific issue by identifying defects or peculiarities in a theory which might lead to new science. By doing so, "why" questions need no theory and may pursue an explanation of observation without reference to a pre-established groundwork.
I think that "how" questions are extremely important in the actual practice and study of science, but "why" questions embody the ever-striving, almost combative quality that peer-reviewed science takes on when theories compete with one another for acceptance and dominance.
A: I agree with Anna V, but it is possible to say it more concisely. 
Physics is a description of how the universe behaves. It does not say why. Never the less many why questions do have answers. 
"Why is the sky blue?"
Because of Rayleigh scattering. 
That is, because a law of physics describes this behavior. 
"Why does Rayleigh scattering work?"
Because it can be derived from the Maxwell's equations. 
That is, because this law is a consequence of a more fundamental law. 
"Why do Maxwell's equations work?"
There is a point where you cannot answer. You can just point to experimental evidence. These are fundamental laws from which we start, like mathematics starts from axioms and derives theorems. Or perhaps, these are the most fundamental laws we have worked out so far. 
"Why does Rayleigh scattering describe how light and air interact?" 
Because air molecules are smaller than a wavelength of light and easily polarized by an electric field. 
That is, because this is what is in the universe. This might lead to more experimental evidence, or how the laws of quantum mechanics determines the size of atoms. 
A: I personally think that why is the motivation behind finding of how? Without why the thrill of science is mostly fade but as ideas grow one understands the importance of how ? Actually the importance of why is much to a enthusiastic learner, but the importance of how is much more to a physicist . But in my idea they are complementary to each other they both contributed and still contributing equally in the creation of this world- the beautiful world of physics and other sciences.
A: As pointed out in this article, physics is where you eventually end up if you repeatedly ask "why" starting with some random topic. Since each subsequent "why" question is going to be harder and harder to answer, this means that once within the domain of physics, you should expect to be able to play this game for a few more moves. Within physics you can give an explanation in terms of deeper theories, but soon you'll end up at the Standard Model or general Relativity and with current knowledge, we can't dig a lot deeper without speculating.
A: Wondering Why is precisely what makes us human, hence since we'll always be human, we'll never stop wondering why.
Now, every Why? can only ever concern itself with the ultimate reason for something. 
However this ultimate reason does not exist, it is an impossibility.
How do I know this? Well, so there are two possible scenarios as to how humanity could seemingly discover the ultimate reasons, however they are both inadequate. The first is that one day some sort of omniscient being, a creator, a god, appears before humanity and tells us these ultimate reasons, however this will only generate even more Whys. The other is that well into the future an advanced civilization gives rise to an exceedingly powerful quantum computer approaching the limit of all the qbits in the universe, which then is capable of interrelating all the mathematical and scientific laws and discoveries, to find evidence of the work of a creator somewhere within our universe who has provided unknowable knowledge that will help us to leave our universe by harnessing all the energy in the universe to travel to another universe with different string vibrations and so different laws, or to even step outside our universe and transcend reality and enter into the nothingness where there are no physical laws, no time, and no space, just the existence of an ultimate reason and possibly an omniscient being. See this would require the soul/spirituality to exist so that we could inhabit such a place. So maybe when we die is the best chance we'll ever really have of finding out the ultimate Why?
Anyway so these two above are pretty improbable, so we'll just have to accept that we'll never know the ultimate reason, there might not even be one, there just is. Anyways so every isolated why is actually just a how, although keeping it isolated from other whys lets us treat it as a why. This is convenient since using why instead of how is more natural to us humans in language. However we should know that every isolated why depends on another why, which depends on another why, and so on, until reaching the ultimate Why, and this my friends is called conditional reality.
This understanding is what gives rise to the disillusionment with science, in that you'll never get to the ultimate truths, even if you know all the hows. Science then as a means for explaining things has no human meaning, the why?, as it's just a whole heap of hows?. Humans don't care about how?, they only care about why?
However science is still very interesting in terms of learning how lots of things work, but once you know enough of the whole picture, well the rest is just finer and finer details and specializing which is just boring really, you’re not learning anything worthwhile, there are so many other more pleasurable and relaxing things you could be doing.
However empirical (science) and systemic (mathematics) knowledge should not be placed higher than semantic (language) knowledge. Without language we would be nothing. Words give us meaning, letting us experience many emotions, and connect with other people. Generally people are the source of happiness in life, so we don't need to go off pointlessly wasting a major chunk of our entire life away concerned only with science and mathematics, unless you're trying to discover something which will benefit other people in which case go ahead. Otherwise enjoy people and languages, movies, TV shows, music, history, and focus only on answering your own personal conscious subjective whys in life such as: Why am I acting like this? Why am I doing this? Why did I do that? Why will I do that in that? for what reasons. Mostly just look inside yourself, once you know enough of science and mathematics, be true to yourself, know thyself, enjoy life's short term pleasures, other people are so important, work towards life's long term goals to bring future happiness, and maintain your health. As this is the meaning of life: to find happiness. Keep life this simple, enjoy the moment, be mindful, ....
