I'm not a social person.  And I think I figured out why.  This link talks about caffeine and how it can actually impair one's thought processes.  The Yerkes-Dodson law in cognitive science basically says something to effect that everyone has a different basal level of arousal or awareness, and this is a function of our anatomy/physiology.  Take a look at the picture:

The article says, "The phrases 'extrovert' and 'introvert', as they are currently used in psychological literature, refer basically to a person's basal level of -- you guessed it: physiological arousal."  So in other words, certain people are naturally on the up-slope of the arousal curve and some people are on the down-slope.  Or some people are at the bottom of the slope and some people are at the top.  People at the bottom (extroverts) generally seek more forms of input.  They can multitask.  They're loud.  People at the top (introverts) are generally quiet and shy away from large groups. 

I'm a textbook introvert.  I don't like to talk to people.  I don't like to make new friends.  I'll go out of my way to not talk to someone.  It's nothing personal.  It's not that I think I'm better than everyone because I'm at the top of the curve.  It's just that I'd rather avoid conversations.  And now I finally have an excuse:  I'm on the border of being overstimulated.  Be careful, you might push me over. 

I kind of thought of this on my own when I was a kid.  I noticed that whenever I went on a roller coaster, I would walk away and talk constantly about how awesome it was.  My conclusion was that I was more sociable because of my increased adrenaline levels.  So that would mean that I normally have relatively low adrenaline levels.  It sorta makes sense.