Because it is good for us and we matter to God!
The burden of carrying around resentments and unforgiveness is like a cancer, which will crush our body, our soul and our spirit. It is we who will suffer; indeed, the instigator of our pain may not even know they have so hurt us and may be carrying on with their own lives, whilst we are stifled in ours. As we hold onto the hurt, we can relegate ourselves to living in the past and not enjoying today.
In life we cannot choose what happens to us, but we can choose how we respond and how we let it affect our life.