I started from environmental concerns, after moving closer to work and first checking into the local bus. After realizing that the bus would turn a 10-minute by car commute into a 45-minute bus commute with a change in the middle, I turned to biking, which I hadn't done regularly for over 10 years. After I did it for a while, learned more about what I was doing, and discovered BikeForums, I became more passionate about it for its own sake. It's made me feel part of a real community of people, those who use bikes for transportation. We've been able to go from a 2-car to a 1-car family (of 4), not only saving money but also giving me the satisfaction of finding a way to live my convictions in a tangible way.
Learning to maintain the bike myself has also advanced my mechanical ability beyond what it was, and of course the fitness is a nice plus too. That was never my primary motivation, because I've always been skinny anyway, but being just over 40, it was certainly getting time that I paid more attention to it for other reasons! (I've ended up with high blood pressure anyway, but who knows, without biking it might be worse. It does run in my family.)