There's a charming little custom among motorcyclists that cyclists should adopt. It's called the Wave, and it's considered poor form to refrain.
I make it a point to wave to any oncoming cyclist I see out on the roads. Of course this rules makes no sense on a bike trail, in a race, starting out on a tandem, or during a time trial. So we're absolved from this bit of etiquette while in engaged in those pursuits.
NOTE: Sure, there are some Harley purists who refuse to return a wave to anyone riding a "rice rocket," but they come by rudeness honestly, as they tend to suffer from various intestinal ailments and are constantly disappointed by the performance and durability of their machines. So we'll ignore them.
Otherwise, it's good form to wave and recognize the other person out toiling along on the roadways, exposed to the same weather, hazards, hills, wind, and dogs as you. While he/she may be pushing a 45 pound iron-framed Wally-World special, it's not your place to judge. Perhaps you started out on one of those heavyweights and gradually embraced high-quality Italian (Taiwanese) Carbon Fiber.
It doesn't matter -- that's another person riding a bicycle and that is a Good Thing. No matter our supposed level in the imaginary hierarchy, there's hardly ever a reason not to wave. Yet too often I've encountered Joe Catthree wannabe seething with rage as he (almost always 'he") passes by on the opposite side, unable to break concentration to wave since he's this close! to breaking that Strava KOM on Superflat Road.
It's beyond silly.
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When you pass someone, it's also good form to say "Hello!" in a bright, cheery way that conveys mutual enjoyment in cycling. It's poor form to motor past someone like Armstrong strung out on Poe and T. I also time my pass when there's plenty of room (so I'm not threading the needle between the other rider and cars). Pas son the left if you live in a country that drive son the right side of the road.
We are a minority. We cyclists are all battling the elements, distracted drivers, narrow, debris-strewn roads, hills, wind, fatigue, and age.
So let's give a cheery wave or "Hello!" and help make the battle slightly less demanding.