Potential and Potential difference My first query is why can't we comment about the potential at a particular point why we always define potential difference and secondly why is potential at infinity considered $0$?
 A: An electric potential is defined as the amount of work needed to move a unit of charge from a reference point to a specific point inside the field without producing an acceleration.
So potential is always defined between two points in a field. Potential as itself means nothing, we only bother about the difference of potential as all the other quantities like work done, potential energy of a charge direction of current only depend upon the difference.
Further, for a point charge the electric potential is given as -KQ/r which is actually 0-KQ/r.
the potential at infinity is taken as zero by convention, but you may take it any point as per your convenience.
Or you can also say that as r tends to infinity KQ/r tends to zero.
In all the questions that you see in which you have to find the potential, it is not the potential at that point, it is assumed that the potential at infinity is
zero and hence you are finding the potential difference between that point and infinity.
A: I am answering with reference with gravitational potential.Same arguments may be extended to electric potentials.
Potential Energy is stored in a system when work is done on it against a conservative force,say gravity.
Why is Potential Energy developed anyway?
Suppose you raise a body by a small height and then release it. It will do some work in falling to the ground. Hence you know that at this height,the body has a potential to do work.Now suppose you raise it to a greater height.It can now do a greater amount of work in falling and hence has a higher potential. So we know that raising the body gives it the capacity to do work and must have some energy -- comes in the picture,Potential energy.
Note that you can measure Potential Difference at any height from the ground by measuring the work done by it in falling.
There must be some distance where the body will not fall back to Earth . This very large distance where the effects of gravity is nil is called infinity. At this point, body will do not fall back to Earth and hence it is at zero Potential.
This is one reason as to why other potentials are defined negative.
However what if I say potential at infinity is say, 2 units.Will this affect my Physics laws.No it would not.It would simply mean that all Potentials I have defined will needed to increased by 2 units and probably mean that 2 is the new zero(pun intended).
As to why we Potential Difference is favoured over Potential , note what I have been doing till now.I am measuring body's capacity to do work at any height with respect to its capacity either at ground or infinity.

P.S. In Physics infinity is a very large distance and not same as Mathematical Infinity.
A: The change in potential energy of a unit positive test charge in moving from $r_1$ to $r_2$ in the field of a (fixed) point charge q is (kq/$r_2$ - kq/$r_1$)  By choosing the reference point at $r_1$ = ꝏ , the second term goes to zero, and you can write kq/r (a single, simple term to remember) for the potential at any other point.
A: For a conservative vector field $\vec{V}$, a  scalar field $S$ can associated such that:$$\vec{V}=\nabla S$$
As you might notice from this equation itself , it is only the gradient which is physically interesting to us. For instance you could add a constant $K$ to $S$ and the given equation is still satisfied since gradient of any constant is zero. $$\vec{V}=\nabla (S+K)=\nabla S$$ . Thus we cannot comment about this scalar field's value at a point since it is defined by the above equation. The potential is such a scalar field whose origin lies in the above equation hence commenting about its value at a point is absurd.
As to why it is zero at infinity , that is because of its inverse dependence on $r$.It is not such a hard and fast rule though. For some charge distributions the zero is not taken at infinity but on the distribution itself as in case of an infinite plane charge.
Can the person who has down voted my answer also comment as to what was wrong with it?  It would be much appreciated.
