I ring my bell because I ride with quickness. Cars in the city spend long minutes idly in traffic, but I can, and do, squeak by on the edge of the road. Often, those cars aren't sitting mindlessly - passengers are getting in and out, drivers are pulling back into traffic - and when I ride by, I can be faster than they think. City biking is an efficient way to move through the world, but motorists seem to think they have a monopoly on speed. I ring my bell to remind them that I am a body in motion.
I ring my bell because I am often mistaken for an entire population when I ride. "You cyclists are all the same," people snarl. "You run red lights, you never signal, and you all ride without lights. You deserve to get hit." This mindset scares me. Drivers share the roads with all kinds of other vehicles - scooters, big trucks, other cars, motorcycles - but for some reason, cyclists ride with a target on our backs. Ringing my bell might do nothing to change the minds of people who resent our presence out there. On the other hand, a bit of eye contact and a friendly smile are the first step in reminding other road users that, behind the wheel/handlebars, there's always a person.
I ring my bell because it's my bike's horn. I do my best to follow traffic laws - like anyone out there on the road, sometimes I don't come to a complete stop, or signal - but when other people are counting on me to give them the information they need to keep me safe, I do it. Some people choose to ignore my safety in favour of inching ahead in traffic, or roaring past me, or even just scaring me for sport. I give an indignant ding of the bell when that happens, because I don't have the luxury of a deep, foghorn bellow.
I ring my bell because, when I ride in a group, sometimes the sheer joy of riding together overwhelms us. It's not the prettiest symphony (cycling bells are designed to be strident and attention-grabbing, so "melodious" falls pretty far down on their list of priorities), but the sound of a herd of bikers clanging together down the road, often with whooping and hollering, is fun and joyful. There's no explanation, no rhyme or reason. We're our own parade.