Develop the ability to forgive.
The ability to forgive, I think can be a huge element in a happy life. It is sadly something that a lot of people fail at. Harbouring a grudge only creates more misery to those who hold them and preventing good relations not only with the target but also with any relationship that may bare some resemblance to that target and yourself.
We all makes mistakes, all of the time so why not have mercy when others do? Remaining angry feels horrible and is a negative waste of energy. However forgiving someone brings a refreshing sensation to the mind and healing to the relationship and it’s one less negative emotion that you need to feed.