I covered a colleague’s class today. I don’t owe him a favor and we are not close. I am doing it because it is a convenient time for me to be doing a favor and someday I may need a favor. I don’t expect to get the favor from him because that would be too much of a coincidence.
There are times when a teacher in our department needs a class covered and generally someone will step forward. The system isn’t fair, in the sense that some people give more than they receive, but it seems to work. I cover classes partly to build up a kind of equity. Someday I may need help and prefer to have paid in advance.
On the other hand, there is a good chance that I will retire without ever collecting what is “owed” to me. I prefer that possibility than to retire owing more than I’ve given.