“Holding on to anger, resentment and hurt only gives you tense muscles, a headache and a sore jaw from clenching your teeth. Forgiveness gives you back the laughter and the lightness in your life.”
When Chazal tell us to forgive our friends [and enemies!] it is not only for the benefit of the forgiven but also for the benefit of the forgiver. The Rambam [hilchos tshuva] says that the way of merciful Jews is to forgive. Hashem gave us a nature that enables us to live less stressful lives.
:)