Insofar as physics answers "why?" at all, it does so through the principles. Such as the principle of relativity, the equivalence pronciple, and the uncertainty principle. These are the things that physicists have found they cannot escape, and which seem basic to the development of physics. A good (and I want to stress GOOD) popular introduction will give you this.
But if after you have read up on Relativity and quantum mechanics you are still unsatisfied, and you want to know why physics is uncertain or relativity is so, rather than something else, then physics has no answer for you. Physics is the study of how nature is, and the study of how she might be is, maybe physics and maybe just speculation.