My reasons were, in order of decreasing importance:

Fitness/Time saving (Days got shorter, couldn't fit 3 hours of biking in the day anymore... eliminate 1.5 hours of car commute and suddenly I have 3 full hours of cycling again!)
Fun
Bragging Rights (I get to say neener, I can bike 19 miles to work and you moan about taking the stairs to the 2nd floor, oh by the way I weigh 150 pounds more than you!)
Saving Money (Up to $1000 per year at my current rate)
Moral Superiority/Advocacy (hey I burn less gas! That makes me better right? Ok maybe not, but eh.)