"Thank You"

I ride the bus here quite often. As people depart the bus, at most stops, one or more people will call out "thank you" to the driver. I do sometimes, but much of the time I'm distracted or shy or not in the mood.

But today I got to thinking: how did this come to be? How does this protocol sustain itself? I mean, it's not like there's an entry in a bus rider's handbook saying you must thank the driver when leaving. But somehow we keep doing it. I think it's a beautiful thing.