Genetics, epigenetics, nature, nurture, heck - past lives or karma, who knows? I certainly don't.
All I know for sure is that in my case, it definitely wasn't nurture. I was never encouraged. I lived in mortal fear that I'd be caught in something feminine, like a skirt I might have "borrowed" from my sister.
But - I also know that as soon as I knew there was a difference between boys and girls, I also knew that I was on the wrong side of that divide.
I've struggled ever since and always will.
Dressing, along with a few other nods to my "girlier" side, helps keep me from having a total meltdown.