When someone feels wronged by another person, the wrongful act stands as a barrier between them, and the relationship is somewhat fractured. You can’t change the past, but what happens next can make all the difference. A sincere apology can bring reconciliation and growth to the relationship.
However, if that apology does not come, then the wronged one will continue to feel hurt, the wedge is still there and grows. Depending how big the wrong was and how often it happens, the relationship can really totter. They may even seek or demand justice, understandably so.