cool question. well, there's always the whole similar interests stuff, but like stacy said, it's more like clicking. i've met people who i have nothing in common with, yet, they're some of my closet friends.
also, proximity. i'm close to alot of people from my high school class, i'm best buds with my neighbor, and i've made alot of friends from my college classes.
though making new friends is not so easy with me because i'm always cautious/apprehensive with new people until i get to know them. i didn't even know about that side of me until one of my friends told me. She thought i was a judgemental asshole when we first met at a summer job, but now, we're so friendly and cool, that we just laugh ourselves silly about it.