There are lots of good, sound reasons for riding a bike as a means of practical transportation. However, St. Louis cyclist Brian Marston gave a presentation at work about bike commuting, and in the process discovered the real reason he rides his bike.
Everyone already knows riding a bike is cheaper and better for your health and better for the environment and fun. Those points are completely obvious to anyone who thinks about it for ten seconds. Trying to convince people to ride by dressing up what they already know with some numbers about fuel saved and emissions reduced and all that isn't all that compelling, if they're not already inclined to ride anyway.
Personally, "I ride my bike to ride my bike" is where I'm at now. I don't expect anyone who doesn't already ride to get that. I don't even expect a lot of people who do ride to understand what I'm talking about. But I hope that someone starts riding, and six months or a year from now they think about it and go, "Oh, I get it now!"
I started riding because I didn't want to pay for a parking pass. Then I moved through the other arguments (health, environment, fun) as a way to justify the choice to myself. Then I tried to use those arguments to justify bike commuting to other people.
But, if you're riding to save money, it doesn't really save enough money to make it worthwhile. I'm planning to sell my car, but it's kind of a wash since I'm also thinking about buying a $2,000 bike and I've bought lots of other bike gear in the last year.
And yeah, I burn off calories by riding, but actually, I'm trying to gain weight rather than lose it, so the biking actually works against me. And even if I were trying to lose weight, at some point you hit a plateau and risk losing interest in biking if that's the reason you're doing it.
And after a while, if you don't love doing it for its own sake, it's kind of a drag to take one for the team and ride a bike to save the planet.
A lot of days, riding a bike isn't super fun -- like when it's raining or snowing or cold or windy or you're sick. But, if you ride a bike to ride a bike, well then you never have a reason not to ride.
For me, it's just a part of who I am. I can't imagine not doing it. And it's nice not to feel like I have to explain it to anybody.