Single speed, like fixed gear drivetrains, are cool because they require no stupid derailers, shifters, and cables. This paraphernalia is a necessary evil on some rides, but in any situation where you don't need variable gears, it's just more fun to ride without it. Simple, durable, low maintenance, efficient, elegant. I've had a single speed bike and liked it, as a compromise between derailers and the simplest possible setup (fixed gear). Eventually, though, I converted it to another compromise that is one step closer to the derailer end of the spectrum: three speed internal gear hub.

When my planned route permits it, I really enjoy riding a bike with the bare minimum. No brakes, no lights, no water bottles, no tools. Just a frame, two wheels, narrow and light handlebar, single sided clipless pedals, fixed gear drivetrain, smile on my face and not a care in the world.