I wasn't as fit as I should have been on a walking holiday. So I got home and thought 'what about that old bike lurking in the garage?' Got it out, dusted it off, thought that maybe as a dim and distant target I might one day be able to bike to work. Discovered that unlike any other sport or exercise I've ever tried, cycling is fun!
Did my first commute about a month later, much to the surprise of self, spouse, colleagues.
Have just moved work site and now have 13.5 miles each way (instead of 10.2). It's more miles, and takes longer, which means more fun in the day.
So I cycle because I love it. And I agree with the silly grin it gives you. Any side-effect (weight down, size down, fitness up, environment saved(!), petrol costs vastly diminshed, etc) is just a bonus.