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.
We may have been terribly wounded by other people’s actions; even hurtful words from childhood can produce guilt and shame resulting in our deep resentment.
Bizarrely, holding onto unforgiveness can be a comfort, as anger can stop us feeling the full weight of our sorrow. Forgiveness is letting go of our hope for a different past, but this can be more painful (for a while) than holding onto unforgiveness.
Even with God's help forgiveness is not easy, but it is life transforming. The perpetrator may not know or receive that forgiveness, but it releases you of that burden. In Matthew 18:21-22 Peter asks Jesus ‘how many times should I forgive a brother?’ Jesus replies, ‘up to seventy times seven’.
Jesus is saying we are not to put a limit on our forgiveness.
It is a process, and we may have to do it consciously every single day and lean on God's strength to do this.
We cannot forget. The hurt and pain actually happened.
However, remember that they, too, have a Saviour.
If it is too difficult for you to do at the moment, ask God to work in you and in time, bring you to that place of freedom. We will never fully recover until we truly forgive.
A few books on Forgiveness worth reading:
Tramp For the Lord by Corrie Ten Boom.
Father, Forgive by Robin Oake.