I can mirror some of the things said here by others.
I never really felt like I fit in with other girls, and that only got worse as I got older. I went through a very girly stage when I very young, but around the age of 13-14 I stopped pretending and really realized that I didn't have much in common with them. They all liked to do their hair, and make up, and I definitely wasn't like that. I wasn't a tomboy, but I also wasn't into all of that. The only thing I really liked, and still like, is painting my nails.
Once I became a mother, it became even more obvious. Being around other parents, specifically mothers, there is nothing connecting me with them. They are like foreign creatures for me. So I never fit in with them either, though I had thought I would.
Writing, and connecting, with male characters, playing male characters in most all of my video games. Being the man in porn, or the "top" in guy on guy porn. Finding myself jealous of the male form. Creating on-line male persona's, the dreams of being a man and not being strange, the time I was very serious about transitioning but ended up ignoring it and spent twelve years trying to be a woman again.
Finally, these forums. They helped connect all of those dots for me.