How come I never hold grudges?

Someone can do something really bad and if they try to make amends i forgive. I dont usually let it stew in me. I forget about it. The only time i hold a continuous grudge is when im continuously being messed around with. Other than that i always forget or try to put it behind me. It seems other people hold grudges longer than I do. Why do they hold grudges over stupid things? Do they like to do that?