I was born with cerebral palsy, and was not very popular as a kid because of it, and was treated like shit by girls in my teens and all through my twenties also because of it. As a result, I was always socially akward, and tended (and still do) to have friends (what few would bother with me) in the same vein.
Even though my mom was the one who introduced me to Star Trek (she was an original Trekkie, and had me in 1970), between that and Star Wars, and the original Battlestar Galactica (which came out when I was seven and eight, respectively), I discovered an early love for sci-fi. As I entered my pre-teen years, I also discovered a love for prose sci-fi, also due in part to my mom, who taught me to read from an early age.
From all this, I figured I was a nerd from birth.
In short, it's all my mom's fault. And the girls who wouldn't date me.
Of course, THAT argument falls on it's ass when you consider the fact that my girlfriend is ALSO nerd with a love of sci-fi.