When the disciples asked Jesus to teach them how to pray, He gave them this model commonly known as the Lord's Prayer.
Pray then like this: “Our Father in heaven, hallowed be your name. Your kingdom come, your will be done, on earth as it is in heaven. Give us this day our daily bread, and forgive us our debts, as we also have forgiven our debtors. And lead us not into temptation, but deliver us from evil. Mathew 6:9-13 ESV
In it, we are to ask God to forgive our debts "as we also have forgiven our debtors". The parallel passage in Luke is similar, but specifically mentions forgiveness of sins:
And he said to them, “When you pray, say: “Father, hallowed be your name. Your kingdom come. Give us each day our daily bread, and forgive us our sins, for we ourselves forgive everyone who is indebted to us. And lead us not into temptation.” Luke 11:2-4 ESV
I was discussing this with a couple friends last night, and one of them mentioned that we do not really want God to forgive us in the same manner that we forgive others, as that would be a much weaker, incomplete forgiveness. In fact, we would much rather pray, "Father forgive us completely--wholly unlike how I am even able to forgive others."
I understand that we are encouraged to forgive others as completely as we can, but I also know my weakness in doing this.
So, again, why are we taught to ask God only to forgive us in the manner that we forgive others? It seems we would want a much greater level of forgiveness--the kind of forgiveness that only God can give.