Our hearts should be free from any form of grudge against anyone. There are weights that we are not meant to carry as christians. An example of this weight is unforgiveness. It weighs us down spiritually. It is not good for us to harbor people in our minds. No matter the wrong they must have done to us we should have a forgiving heart.
When we forgive we are able to move ahead. Every past hurts should be left at the past because that is where they belong. We shouldn't carry them along with us to the present. If God has forgiven us liberally who are we not to forgive?
Forgiving doesn't mean we are weak. It doesn't make is the foolish one. Even if the person did is really bad we should leave the vengeance to God. We shouldn't take it to heart or look for means to retaliate. God told us to love our enemies as well. There's no directive that says we should hate anyone.