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We know that two mass particle attract each other with a force $$F~=~\frac{G M_1 M_2}{r^2}.$$

But what is the reason behind that? Why does this happen?

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Is there a particular part of the equation that you are asking about, like are you asking where say G comes from? Or why all mass attracts? – DJBunk Jul 24 '12 at 20:35
I am not asking about the equation.I am asking why all mass attracts. – orange Jul 24 '12 at 20:46
If you have a physics background Zee in 'QFT in a Nutshell' gives a rough explanation of this in chapter I.5. And Peskin mentions this on pg 126. Otherwise I don't know of a good heuristic explanation for this. – DJBunk Jul 24 '12 at 20:58

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One could explain "well, gravity is the curvature of spacetime due to the mass-energy". But that would only lead to "well, why does mass-energy curve spacetime?" And, should someone produce a proposed answer to that, the follow-up question would have to be "but why is that so?" etc.

At some point though, one must accept that there are genuine fundamentals, genuine primaries that cannot be explained in terms of something "more" fundamental, "more" primary.

Gravity is considered one of those fundamentals. But the question "what is the reason for gravity" presumes that gravity isn't fundamental. So, the only proper "answer" to your question is "to the best of our knowledge, gravity is fundamental".

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