It's also easy to forget that there are people riding bikes who don't actually know HOW to shift. I have taken people out on rides (lending them a bike) and they actually don't know how to get to an easier gear - and it isn't intuitive that small in front, big in back is the easiest combination. Gotta learn this stuff.

More unfortunate are those whose bikes may not be working properly so every time they shift is like clunk, clunk, slip, catch, clunk, etc. So they stop fighting and just find a gear that doesn't screw up. then they end up mashing. I always feel awful when I pass someone who then tries to shift to catch up and the bike sounds like it's exploding to pieces.