Personally, I had kind of a... different childhood. I was constantly changing schools, I've gone through the new school experience more loving times than I can think of, meaning I've been thrown into an environment several times in which I am not at all comfortable with surrounded by people I don't know. I'm not very outgoing as it is, but to constantly be in a new school with an entirely new social scene has effected me in that way, I think. Now I am hardly social at all; I rarely hang out with friends unless I'm pretty close with them, I've never had a girlfriend, I even had my boss tell me at work I don't talk much.
I think your sociability is partly your personality and even moreso your circumstances growing up. Your childhood environment shapes you.