Congruence
http://jennalynne.wordpress.com/2008/09/04/congruence/9/4/2008
The reasons why people crossdress are complex. I wouldn't want to oversimplify. It would be foolish to claim, for example, that all painters paint for the same reason. Various forces may be at work in various people, or in the same person at different times.
Many of us, though, start out by feeling feminine on the inside. We set out to try to make the outside match the inside. We're seeking congruence.