When is a relationship broken beyond repair?

Relationships can be severely strained, fraught with anger and frustration, and perhaps put on ice for weeks or months or years. Yet in the end, the two people can often reconcile in some way, so that they can enjoy a genuine (even if not deep) relationship again. In some cases, however, that's not possible. Why not? In such cases, must the problem be that one person (or both people) continue to behave badly? Or might reconciliation be impossible between two good people? If so, why?