Self-hatred. They simply radiate it.
And they are the most acutely self-conscious people you'd ever want to meet. Odd, when you consider they claim to only "feel like themselves" when posing as the gender they are not.
What they get out of the lifestyle, I don't know. It certainly doesn't appear to be sex. They may give a lot of it, especially if they're working as prostitutes, but they sure don't seem to get much or really even be after that as a main objective. They're seeking some level of affirmation of their externals that most people aren't capable of giving them, and they're willing to sacrifice any semblance of normalcy or dignity in order to get it. There's every reason to pity them.
I really don't know what makes them tick, but I do know that to call it "biological" when it clearly isn't is a copout.