Why do some people like to hold grudges?

holding a grudge can be addictive.... for some it feels good to hate...

Perhaps they aren't convinced of your sincerity. Forgiveness requires both contrition and restitution, both of which must be genuine, and even then, forgiveness is a gift that must be freely given. The contrition and restitution are given just for the possibility of receiving forgiveness, but they don't guarantee it.

The people I hold grudges against have never apologized. If they did, I would probably accept their apologies.

What's it over?

They like psychic pain.