Had my ex over for dinner last night. We were together for 8 years and that ended 14+ years ago and have seen each other regularly.
But occasionally a kind of disagreement happens which rings up emotional memories of all the crappy arguments we had when we were a couple. Happened last night. We got through it and past it but it's one of the cons of knowing someone a long time.
The same kind of familiarity, when pleasant, is comforting. But when a conflict today awakens feelings of older strings of such stuff, it has a particularly unpleasant echo, even years later.
It's one of the reasons that long-term relationships break up, when the latest conflict remains connected to a long history of them and there seems to be no hope of stopping the cycle. It can lead to relationship despair.