I socialize because there is nothing, nothing, like the feeling you get when you're riding in a car to who-cares-where with close friends you feel perfectly comfortable with. Laughing, making good natured fun of each other... that's happiness.
There are several social groups I count myself as a part of. My main one is mostly made up of acquaintances, but I have a few close friends too. People are always drifting in and out of the picture, and I try to be friendly to every one of them. Because I never know when the new friend of a friend is going to turn into one of the most important people in my life.
It's hard finding groups like these. The only advice I can give the posters here is to always look for good excuses to talk to people. I know I'm not good at talking to strangers when I haven't been introduced to them. But a few weeks ago I randomly complimented a girl in my class on her choice in band t-shirt, and we've been talking ever since. Which is nice.