How do we choose our friends?

I have a few conversations over the weekend with different people about different relationships. One person has stopped being friends with a couple of people and another is frustrated with a long time friend. I have been thinking about the nature of friendship. Why are we friends with people? Not why do we have friends - that is clear to me but what is the reason we are friends with certain people. Are we friends with people because we have fun with them? Are we friends with them because they need us? Because we need them? Both? All? Do we choose friends or fall into them?
I have my friends from school all of whom I really like but i likely wouldn't be friends with any of them if we weren't in school together. Will I keep in touch with them if I end up somewhere else next year.
what is more compelling convenience, history, shared interests
Not sure I know and I am not sure it really matters
I should get back to finishing up my work- tomorrow is my last day of finals and I have assignments as well. After that I have to figure out my summer plans and hopefully I will figure out next year as well. I also have to devote some time to movie stuff. Now I have to devote time to memorizing the anxiolytics. Maybe i'll spend some more time on these random fairly useless thoughts later.