Mainly because it's a cheap form of great entertainment, but it's also healthy, environmentally friendly, and it gives you a somewhat unique perspective on life. I voted all of the above even though I don't really "have" to. I'd feel guilty if I didn't commute by bike, but I still have a car that I don't plan on selling. There are times when I go 60 miles to get a new vintage bike which wouldn't be feasible to do with another bike.