When we sin against God, we often go to Him in penitence asking for forgiveness. And we tell God with all sincerity: "I promise You that I won't do it again." God, knowing the future that we're likely going to commit the same sin again and again (and we actually do most times), still forgives us as though He believes us that we're not going to commit that sin again.

This is how God expects us to forgive our spouses when they offend us repeatedly.