Pretty much all the reasons that I commute have been touched on already.

Mainly: I live 3.5 miles from work, it's faster, it's fun (driving just makes me angry), and it's a cheap way to get my weekly mileage up!

It's worked out so well since moving to the Bay Area that I've sold my car. If you can ride to work, to the store, and to baseball games, what more do you need?