Psychologists often claim that when ex's run into each other again, there is a 70% chance they will get into bed together "for old times sake" just to check whether there is any feelings still there for each other or not.
Sometimes, with the passage of time since the breakup, they can see that they did not have good enough reasons for "chucking it in", and they may want to get together a few more times just to be properly sure the relationship cannot be salvaged.
Having a few more fucks can help to forgive each other for the breakup, so that the emotional baggage can be dropped away......to allow each to no longer have guilt feelings.