Sometimes people change over time, or due to life events, and this leads to reconciliation. This is especially the case with blood relatives, who have a bond whether or not they are on good terms, and close friends who fell apart.

In marriage breakdowns, people can also resolve their differences and get back together. However, sometimes, when that happens, they find that they have both changed and are no longer suitable for each other.

Anyway, it is good to make peace with people (as long as there are no extenuating circumstances, eg concern over safety, continued misbehavior, etc) and to let them know the door is open to reconciliation, if they ever want it. Allah guides people's hearts, and you can also pray for reconciliation. However, in cases of marriage breakdowns, it is better not to hold out too much hope; you can pray but you should also accept the reality as it is and go on with your life.

Sometimes it is only after a person passes away that the other person is able to forgive them or let go of things. All mu'mineen reconcile with each other in jannah.