I think it's so cool that so many people put down recreation and fun as the primary reason.

I think that people who don't commute by bike often think that when we ride the bike it's like putting on a hairshirt everyday and that we swallow this bitter pill because we're too cheap to pay for gas or we're only doing it for an environmental crusade or whatever when, in fact, once we start riding we love it so much that even if we started off riding to save a few bucks, shed a few pounds, avoid the traffic jams eventually we're just doing it because it's so much fun.

I can't underestimate the power of the above. I like being a little bit a part from the madding crowd- nothing seems to do that like the bike.