As I explained elsewhere in this thread, the special relativity theory is a direct consequence of all the experimental evidence that existed on the electromagnetic phenomena. Formally merging Maxwell theory with the classical mechanics yields special relativity.<p>General relativity is an application of a well known extrapolation heuristics - the simplest possible generalisation of a model. The other possible generalisations were much more complex, and the nature for some reason tend to favour simplicity, so it was reasonable to go that way (as well as it is very reasonable now to go out and test all the possible extensions of the Standard Model, starting with the simplest ones).