This might be an oldish article, but just thought this might be a good time to dig this up.
Jimmy Wales' answer to "Why do people contribute" is especially interesting
"...It's realizing that doing intellectual things socially is a lot of fun—it makes sense. We don't plan on paying people, either, to contribute. People don't ask, "Gosh, why are all these people playing basketball for fun? Some people get paid a lot of money to do that."
He also says "It turns out that people aren't as horrible as the Internet made them seem for a while."