I heard that question a lot, for a while. I spent about 2 years riding nothing but fixed, even on a mountain bike, and people tended to be confused about that choice.
The basic reason was that I had become somewhat bored with the bike, around town. I wasn't mountain biking on a regular basis, and commuting/riding around town had lost a lot of its challenge.
One day, on a 30-mile ride on my cross bike, I realized that I couldn't remember changing gears since leaving the house. I decided to see if I could get through the rest of the ride, comfortably, in the gear I was in.
Sure enough, I got to the house still in my 42x17 gear, which happened to be the same gear I was running on my fixed-gear. At that point, I figured I might as well just ride fixed, and not carry all of the extraneous drivetrain parts around.
And so, I became a fixed-gear zealot. I rode fixed with guys on geared $5,000.00 Serrottas and carbon TREKs, and stayed in my usual position in the pack. I went mountain biking (on the easier trails, I admit), and enjoyed the challenge of getting over obstacles without smacking a pedal.
And, everywhere I went, someone would ask , "Why?"
I didn't ever have an answer that suited them, but I did have one which worked for me: "It's more fun to me, right now, than a geared bike."
Last year, I made the conscious decision to start riding with multiple gears again, as I was getting back into mountain biking and 100-mile road rides. I found that adjusting to the freewheel for those rides was difficult, if I had been riding fixed 90% of the time. So, I rode the freewheel 90% of the time, and rode the fixed, occasionally, for fun.
And that, after all, is why...