And out of pity for him, the lord of that slave released him and forgave his debt. Matthew 18:27
The King forgave his slave so that he could live in peace with himself. The King had pity for the slave. The King forgave in order to better his life and the life of the slave.
In the same way, we need to take care of ourselves. We need to forgive one another, so that we can be at peace with ourselves and with God. If we don't, we hurt ourselves more than anyone else.
There is a story of a little girl who had a fight with her sister. As the day passed, she refused to speak to her sister. At bedtime, their mother said, "Don't you think you should forgive your sister before you go to sleep? The Bible says we should not let "the sun go down on our anger." The little girl thought for a moment and replied, "But how can I keep the sun from going down?"
There are many people that like to nurture resentment. Holding on to resentment hurts a lot more than it hurts anyone else