I have this theory that every time I pick up a grudge, it weighs me down. And seeing as I have a broken back, everything that weighs me down, increases my pain (I actually think that's true for my other pain too...).
A few years of collecting grudges soon builds up to quite a load on my shoulders, and it pollutes my mind and makes it really difficult to enjoy and appreciate the things that are good in my life today. So I take out the trash regularly. If I find that I've picked up a grudge or ten, I choose not to bring them along. Whatever it is that is irritating me, I can usually find some explanation that allows me to let it go. The other person had their reasons, and quite often it has nothing to do with me. Maybe they are under a lot of pressure, maybe they have been through some really tough times? If they did something genuinely hurtful, I am thankful that I have the opportunity to learn from their (bad) example and choose to act differently than they did.
Sometimes it takes quite a lot of adjustment to manage to let something go, cause grudges have a way of sticking, but then again, I realize I may just be projecting. And carrying grudges around is not something I want to waste my energy on, so one way or another, I'm letting them go.