For many years now I've been an avid cyclist. I've raced bicycles, commuted by bicycle and fixed bicycles for a living.
I started out with the usual single-speed kid's bike with a coaster brake and training wheels, worked up to bmx-type bikes and old ten-speeds, and finally, in high school, got a mountain bike with shifters on the handlbars.
Years later, when I was "grown up" with a kid of my own, I graduated to grip-shifters and triple front chainrings, giving my mountain bike 21 gears or more. On my skinny-tire road bike, I had shifters that were integrated into the brake levers so I never had to move my hands from a riding position. The rear wheel featured a cluster of 9 cogs, each one accessed in crisp, accurate clicks by a twitch of my fingers. My bikes had grown progressively more complicated, from heavy steel to aluminum and carbon fibre, with more and more gearing options.
Then, somehow, something changed.
Out of curiousity, I built up a mountain bike with only one gear. I used a catalog-clearance frame and spare parts, and made it as cheap as possible, with the intention of having a no-maintenance city bike.
But I made the mistake of taking it off road. I was hooked on "single-speeding". No shifters, no shocks, just muscle, skill and a willingness to occasionally walk the bike uphill. It was good, muddy fun.
Then came the fixed-gear road bike.
Single-speed bikes are fun, like a cruiser or bmx bike. It's easy to understand how, if you're not in a hurry, the low cost and durability of a super-simple bike can be appealing. It's pretty easy to understand, after all, and pretty easy to explain.
Why anyone would want to ride a fixed gear, on the other hand, is much harder to explain to the uninitiated. A fixed gear bicycle has only one gear combination, it's a type of single-speed bike. The "fixed" part is what makes it different. The rear cog is screwed directly onto the rear hub with no freewheel mechanism. Without some sort of ratcheting freewheel, the rear wheel cannot move unless the chain moves too, and the chain can't move unless the pedals go 'round, and the pedals can't go 'round unless your legs are moving too. In short, you can't coast.
Nonstop pedaling isn't that bad. If you watch racing cyclists, or even serious recreational riders, you'll see that they hardly ever stop moving their legs. You won't go fast, after all, if you keep letting your bicycle slow down. The tough part about riding a fixed-gear bike, or fixie, is that there is only one choice of gear ratio. That means that if you have a low enough gear to get up a hill, you're going to have to spin like mad going down. On the other hand, if you have a gear ratio that lets you spin comfortably going fast on flat stretches, or down hills, you need to do leg presses to get up a hill.
As hard as it can be to ride a bicycle like this over Central Jersey's rolling hills, it's even harder to explain to non-cyclists, or sometimes even other cyclists, why this is fun. Why throw away a century of innovation and technological adancement so you can work harder to go slower? Why ride a silly, risky, evolutionary throwback like a fixed-gear bicycle?
Probably the best answer that I, or any other fixie aficianado, can come up with is the same answer that people who hike the entire Appalachian Trail, run marathons, swim the English Channel or enjoy any of a thousand other difficult, dangerous and unneccesary challenges give:
"Because I can"
Wikipedia article on fixed-gear bicycles