Look down from heaven and see
from your lofty throne, holy and glorious.
Where are your zeal and your might?
Your tenderness and compassion are withheld from us.
But you are our Father,
though Abraham does not know us
or Israel acknowledge us;
you, O LORD, are our Father,
our Redeemer from of old is your name.
Why, O LORD, do you make us wander from your ways
and harden our hearts so we do not revere you?
Return for the sake of your servants,
the tribes that are your inheritance (Isaiah 63:15-17).
Isaiah speaks of God's compassion being withheld. Then he shows what happens when God withholds mercy: We wander from God's ways and our hearts become hardened and we do not worship God. So the reason I need God's mercy is not just to be forgiven, but more importantly so that my heart will not be hardened against the greatness and goodness of God.
I'm also reading the story of the Exodus. After one of the plagues, Pharaoh actually acknowledged he had sinned: "Then Pharaoh summoned Moses and Aaron. 'This time I have sinned,' he said to them. 'The Lord is in the right, and I and my people are in the wrong. Pray to the Lord, for we have had enough thunder and hail. I will let you go; you don't have to stay any longer'" (Exodus 9:27-28).
But as soon as the crisis was passed, he was unchanged in his opposition to God: "When Pharaoh saw that the rain and hail and thunder had stopped, he sinned again: He and his officials hardened their hearts. So Pharaoh's heart was hard and he would not let the Israelites go, just as the Lord had said through Moses" (Exodus 9:34-35).
How much of this kind of religion exists in our churches? Indeed, how much of it exists in my own heart? It is common to seek forgiveness from God - even to acknowledge sin. But that's not all there is to God's compassion. When God's mercy is poured out it will soften our hearts to love him and fear him and obey him.
[Update: Paul Tripp says something similar (but better) in a post on Psalm 51: Something Bigger]