Quoting Chris Carrier, who forgave an assailant who left him for dead:
There is a very pragmatic reason for forgiving. When we are wronged, we can either respond by seeking revenge, or we can forgive. If we choose revenge, our lives will be consumed by anger. When vengeance is served, it leaves one empty. Anger is a hard urge to satisfy and can become habitual. But forgiveness allows us to move on.
There is also a more compelling reason to forgive. Forgiveness is a gift - it is mercy. It is a gift that I have received and also given away. In both cases, it has been completely satisfying.
Source: Johann Christoph Arnold - Why Forgive?, pp.85