: : Great, mass curves space-time and causes gravity, WHY?

: Strictly speaking there are three things which are the source of gravity: stress, energy and momentum. These three things together form an mathematical entity called the stress-energy-nmomentum tensor. Why? Nobody knows why ... at least not in a general relativity sense. I don't know about string theory.

This is what causes it, and the amount it causes, not WHY!

: : Distance is measured differently to an object with velocity relative to a reference, why?

: It's due to the way we measure distance. We assume the postulates of relativity are true. From that we deduce by logical reasoning (and, using a bit of math), that when we measure distances experimentallly then there will be a velocity dependance. The validity of the prediction thus rests on the validity of the postulates and experiments. Since the postulates are formed from observation (i.e. experiments already done) the stregth of the postulate rests on the ability to predict new phenomena.

Again, what and how, not why.