Personally I think common courtesy is just about dead in contemporary society these days. It's not just exercise, it's everywhere. I used to initiate greetings, but the responses were so rare that I just don't bother anymore. I'm quite happy to return a greeting (assuming I see it), but I'm rarely in that situation.
I will, however, stop and assist a downed rider at any time. It doesn't really have anything to do with hoping they'll help me one day, it's just because I perceive it to be the right thing to do.
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I am clinically insane. I am proud of it.
That is all.