Edit: Let me see if I can make myself clear. We know that c is constant no matter how fast you are moving. If c is constant then it can be derived that time slows down and distances get compressed, etc. etc. etc. Einstein derived all of this by realizing that c is constant in all frames of reference. Now, if I ask "Why does time slow down?" and I get the answer "because c is constant" then we are just going in circles. It seems that that is what is happening here. Correct me if I'm wrong but are you not using a derivation of the fact that c is constant as an answer? I'm looking for a more fundamental reason as to why time slows down. Which is the same as asking why c is constant.