There is no "all-encompassing explanation" -- a myriad of factors, both organic/genetic and situational/experiential/behavioral, lead us to do the things we do. Humans like order, not chaos, and want answers, not open questions.
I think there comes a point for any CDer at which you have to let go of the "why" questions. Because, one, you will never have a satisfying answer and, two, the more important questions are what it means to you personally, and how it is to be successfully integrated into your life. Accept your desire to crossdress as a given, and concentrate on making it a positive, enriching, and fun part of yourself. That's a daunting enough task without agonizing about why the desire is there in the first place.