Question 25 of 365; Why do people hold grudges?

The world is complicated, aggravating, and confusing but no matter what I do nor the amount of good deeds I perform, I will always believe I owe the world because it gave me something so much greater that I can never repay, my life. I can chose to be angry at the world and complain about the struggles I face, such as the work load of homework, the stress of trying to please everyone, and a couple of personal altercations but I'm not mad. Anger is a waste of energy, yes I get mad, but never longer than 10 minutes. When you get angry for a long time, you think about the situation and get fired up all over again as like time did not pass. You trigger sad and upsetting memories and all of those emotions hit you again. It's easier to be happy even though it sounds hard. I don't hold grudges because it creates hate. Hate.  When people say that word it really strikes my heart. I strongly dislike the h word with a burning  sense of passion that is no longer belongs in my vocabulary. You know the saying "When life gives you a hundred reasons to cry, find a thousand reasons to smile". True quote. I'm not saying you should ignore your problems. I'm simply saying you need to face your problems but never let the things that make you angry allow you to overlook the things that make you happy. So why do people hold grudges, stay mad for weeks/months/years, and hate? Something I will never understand.