Actually I'm not against the notion of friendship as some form of mutual usery. The only caveat is that the opportunity cost of trying to keep track of that debt and actively trying to negotiate is far higher than anything that you would actually gain from the friendship itself.

You are right that most friendships are based on circumstances, but without accepting my principles, do you have any kind of explanation for why friendships are based on circumstances? My explanation is pretty simple. The main benefits of friendships come from random and unexpected happenings that result in a single friendship having many unanticipated (and often unrecognized) benefits, often occuring long after the actual friendly interaction itself.

Because of this, I argue the phenomenon that gave rise to humans was heavily weighted so that we would tend to enjoy friendships for their own sake rather than needing to be able to calculate and quantify the benefits. Why else would humans seemingly waste so much time on friendship without expecting tangible benefits? Why else would humans care so little about specialization and optimizing friendships? Maybe I would have an easier time coming up with reasoning if I had some alternative exanation to compare with.

Have you ever read the classic literary masterpice "Green Eggs and Ham?" The audience doesn't always know their own taste. If all he wanted was immediate and tangible rewards, then Sam-I-Am never would have offered green eggs and ham in the first place.