You underestimate how easy it is to get someone who's into fly fishing to talk about fly fishing. You don't need to have known them for more than thirty seconds.

Even NYC has a fishing meetup group with over 1000 members.

Good point. Fixating of the fly-fishing example is silly to begin with but yeah- if you don't know a guy, it's certainly an opportunity to meet one.

It's harder to get them to stop.

I love when I get someone to talk about something they clearly love, and they're giddy with joy and struggling to contain themselves. It's one of the finer pleasures of talking to strangers and not machines.

Not everyone live in a big city.