I've gotten to be the unintentional wheel sucker on a handful of occasions-- I'm headed where I'm headed, closing on a lone rider, and when I get within talking distance of them, their pace suddenly jumps up to match or even slightly exceed mine. They will not be passed. I've experienced it on hills, flats, wherever. I had one guy just absolutely tucker himself out on a long, ~2% grade where he just refused to let me by for about 1.5 miles. Then he just turned down a random side street because I think he was out of gas.

I ride solo 90% of the time, I'm not out to steal someone else's effort. But the roadie mentality is strong in most of the riders out there-- they refused to be passed by anyone, much less the old guy on a cross bike.