100 miles into my first 600km brevet, it was 100F, I was alone in the middle of nowhere and had all but decided to quit at the next control. Crawling up a climb, I heard a Harley coming up behind me. The engine slowed as it approached, and i figured nothing good would come of this. The dude came alongside and matched my speed. I ignored him. I'm sure i looked like crap, between road grime, Gatorade stains, and sweat. Finally i looked over. Young guy, beard, no helmet, not exactly friendly looking. He said, loud and clear, "You are bad ass", and roared off. That turned my mood around entirely. "I am bad ass", I thought. I didn't quit at the next control, and finished my first 600k. The next year, I did another super randonneuring series, went to France, and finished Paris Best Paris. Not to be too dramatic, but I do think that boost on the road in rural Illinois was a turning point for me.
I've had a few less friendly interactions with people on motorcycles, but that's just people being who they are.