Debts have power, and that's especially true in a community as closely knit as a Ghanaian parish family or a 1st-century Palestinian village. If I owe you something, you have power over me--not just because of what I own you but because of what that relationship imbalance does to my relationship with the whole community. Those debts can be financial. They can be debts of honor. They can be debts on convenience. But, when I have something to hold over you, it gives me power over your life.
Praying the Lord's Prayer affirms our belief that we cannot know what it means to be let go of our debts until we let go of what we hold over others. We cannot know forgiveness unless we forgive. We cannot internalize (and maybe it goes further than that) what it means for God to release us from our sin until we are as free, liberal, and generous with forgiveness.
Here in Ghana the poverty line is around $1 per day. Imagine, then, what it means to owe someone a debt of even modest magnitude. How would someone ever pull themselves out from under even a small obligation?!? Forgiveness. We live in the poverty of the human condition. What we have to offer God is a pittance. Forgiveness is our only hope. I think Jesus' words made sense to his audience in a way that rich, credit-card-carrying Americans like me have a hard time grasping. What I (and I suspect others like me) don't realize is just how absolute the connection between a willingness to forgive and an appreciation of one's own forgiveness really is.