We are all Serotonin junkies.
Here is part of an article I found explaining why we crossdress.
Crossdressers comprise about 15% of the male population and are normal, healthy, heterosexual men with one little tweak. Their brains are hard-wired so that when they wear women's clothing they produce Serotonin and other neurotransmitters. This gives them a sensation of well-being, pleasure, sexual-gratification and self-identity. Since it affects the reward center of their brains they often feel a NEED to crossdress.
I think we can all relate to this. I wonder if my doctor would write me a prescription to dress?