This differs from what I had hypothesized was the standard model. I think I like my hypothesis of the standard model better than my understanding of your model, so I'll mention it here, on the off-chance that you might also like it.

I think that most people make (or intuit) the calculation "If it's not too much trouble, I should help this person one time. If they are appropriately thankful, and if they do not inconvenience me too much, I will consider helping them again; if they reciprocate appropriately, I will probably be friends with them, and engage in a long-term reciprocal relationship."

In this calculation, 'appropriately thankful', 'inconvenience me too much', and 'reciprocate appropriately' are highly subjective, but this model appears to account for most stable relationships. It also accounts for guilt-trip based relationships being unstable. The "I should help one time" clause may make the world a better place in general, although it is unclear if that's why most people hold it.

It is possible that when you say "social contract" and I say "reciprocal relationship", we mean exactly the same thing.