"Have mercy on me, O God, according to your steadfast love; according to your abundant mercy blot out my transgressions." By praying this psalm, we are admitting that we can ask for mercy from God not because we have earned it but because God loves and is merciful--the nature of God rather than the deservedness of us.
Psalm 51 is one of only seven penitential psalms. I'm wondering why only seven. How often do we need words to express our recognition that we need to be forgiven?
Interestingly enough for me, as I am reflecting on this psalm, I'm also doing the weekly laundry. Those clothes really were dirty, and they really did need cleaning. Soon, they will be presentable for wear once again. The metaphor fails though because the washing I am doing to the clothes will remove only the surface stains.