Forgive a debtor
We must give in order to receive. You cannot receive love if you do not give love.
If our hearts are such that we can only see how others have hurt and offended us, we cannot see how we have offended God and so we find no need to seek forgiveness. If we are always calculating how much this person or that person has violated our rights then we will not, by the very nature of things, be able to pray this prayer.
Forgiveness is the only thing that breaks the cycle of retribution and releases the grace of heaven among human beings.