Initially (in 2000), I had a beat-up '89 Civic that needed to be on the road less because I didn't want to replace it. I was also kind of a chunk (expected of a sysadmin, I know, but still not healthy). So I bought a bike and started riding to work. I replaced the car (with a '00 Tacoma) a few months later anyway, but it would sit for weeks at a time as the addiction had already started. Pretty soon, I was going most everywhere by bike (dates, jobs, church, everything but Home Depot). FF to 2007 and my new wife and I decided to sell the truck after a trip to Europe. I also don't like paying for gas/insurance/maintenance/parking/etc., it's a heck of a lot of fun, and cycling is only slightly slower than the train (but probably faster than the bus).

But the original reason was to spare the Civic and lose some weight.