Though it is talking about birds flying as opposed to someone wanting to be the other sex, I think this game gives a pretty good answer at the 3minute and 15 second mark:
I finally have an answer to your question, "Why do you think that birds fly?"
Birds fly because they want to fly.
They don't need a reason, even if their wings may snap and doom them to die.
They don't fly for anyone's benefit. They don't fly because they were ordered to.
Birds fly because they want to fly and for no other reason!"
...so that is your answer.
Yes. BECAUSE THAT IS WHO I AM!