We're social animals
Much of what's happening on the web today is hard, even impossible, to explain in terms of rational economic behaviour. The voluntary and collaborative work we do will never make us rich -- when we help people, answer their questions, offer web services without even ads.
So why do we do it? Well, Tom Evslin suggests an easy answer. We do it because we can. "We're programmed to want to help and, given the opportunity, that's just what we do. A lot of Web 2.0 is about giving us that opportunity."