Many relationships involve relationship-specific investments - in which case, the kind of "insurance" you're talking about actually makes a lot of sense. You don't want the other party to break the relationship off on a whim, so you expect them to make some implied pre-commitment or you wouldn't even get involved in the first place. This is a kind of cooperation, in that you're solving a coordination problem, but in practice it works more like a Stag Hunt than a PD. Because as long as the relationship works and the stakes are reasonably equal, there's no reason to deviate.