Especially your parents: do you like them / get along with them? Was it always like that? Also: did you show a marked preference for playing alone in your kindergarten years, or did you try and fail at being accepted by the other kids?
I'm fairly sure that the relationship you have/had with your family has a lot to do with wanting or not wanting social contact later in life. The thing about playing alone in kindergarten, though, I'm not sure if it has any link. (I was one of the kids who preferred to play alone.)
I can say with confidence that I don't particularly like my parents. Today I'm mostly neutral towards my father, and I manage to get along with my mother, but still, just being in the same house as her is draining. I really didn't like and didn't get along at all with them until recently.
My mother has a tendency to be the emotional victim of everything, and my father has a tendency to do nothing other than defend her whenever she's involved in any situation. Neither of them seems to understand the concept of privacy. Overall I think they were mediocre parents, and very incompatible with me. I knew it very well when they were wrong, but they would never admit that I could be right.
My brother and I get along without a problem. We were never very close, but never very had too strong negative feelings towards each other for long. My sister, on the other hand, for many years alternated between best friend and worst enemy. If she's not borderline, she's at least "borderline-ish".
After she deliberately decided to meddle in my life (when I had told her not to) and ruin all the support I had at a very delicate point in my life, I stopped talking to her altogether. She still refuses to apologize and doesn't seem to understand why I wouldn't talk to her. I don't know how much, but I think this long history of conflict with my sister also contributed to my isolation and adaption issues.