I do it because I want to, because I want to get stronger, to be independent, and because you connect with people and places on the back of the bike that you can't do from a car. For better or worse, it's exhilarating, exciting, and just plain fun. Plus, it's great to get the juices flowing before work.
Looks like I'll have to get a poncho for the ride home though, it's getting dark and rainy outside.