It can become easy to hold grudges and hard to forgive. In order to be happy, you need to learn how to let things go. Holding onto pain will just keep you from moving on. If you are constantly thinking of the past how do you expect to be active in the present or reach the future?
Forgiveness is powerful, it can allow you to accept joy and rid yourself of hate. Hate can wear you down and leave you using your energy towards the negative rather than the positive. Therefore, the more you let go, the freer you become. Let go rather than simmer in anger.