I can't point at some defining presence or lack thereof, event, "fault" or other "reason" for me being me other than that I'm wired this way. When I was four I wanted to be a girl and that's never stopped. I've always had feminine traits, been more drawn to girls' activities as a kid, never was the typical boy. As an adult I've had considerable opportunity to use that softer side in positive ways - as a single parent, the Mom role fit me as well as Dad.
If there was/is some external influence, the only theory I've seen that kinda fits is that I was exposed to DES in utero and there's apparently a higher percentage of GID among male DES kids. DES is synthetic estrogen, was used to help prevent miscarriage. The theory is that the higher estrogen levels affect brain development in the male fetus... presto, my toenails are in pink polish today.
