Bottom line, I don't know why I am a woman. Perhaps because I can't stand being a man?
Thing is, I don't identify with so much of the "born this way" talk. Maybe there was biological determinism to why I ended up where I did, but I didn't always know this. When I was born, they dressed me as a boy, gave me boyish toys, and sent me outside to play with the other ones. I believed my parents concerning my gender for the same reason I believed them about religion, how to tie my shoes and what time to go to bed. My capacity for skepticism and analysis was still many years in the offing.
I don't disbelieve at all those who say that they've always known. I am a bit jealous of that certainty though. I would say rather that I have always struggled. Paraphrasing Jameson Green, coming out for me was less like coming out of a closet and more like lighting a series of candles in a dark place.
I only mention it, because when I was first looking for help, I ran into so much certainty that I figured my doubt-ridden existence must have been something else. Well, it isn't. I'm a trans as the next person...it just took awhile to figure it out in my case.