There are three reasons for this. One, He has a book and he already knows how others have wronged us. Two, we don't remember things precisely and we'd probably get it wrong. Three, we'd use it to justify ourselves. But no matter how others have wronged us, we are not justified in our sin against them or against God. Only Jesus and his redeeming work on the Cross could ever do that.
Therefore, rather than focus on how others wrong us, we must focus on the depth and gravity of our sin against God--the true, perfect, and just God. At the same time, we must also focus on the heights of God's grace--our loving, merciful, Abba Father. There is something subtly beautiful here: The more we understand the depravity of our sin, the greater we can know the majesty of God's steadfast love.
And yet, we cannot have one without the other. That is, without God's grace we wouldn't dare know ourselves. We couldn't. We'd rather die than go there. But, without sin, God's grace would be nothing. Without cost, there is no value. How can you pardon the innocent?
So what do we do? Look to the Cross. It's all there. At the Cross we know both our sin and God's grace. We know our sin was devastating because of the cost that Christ paid crucified on the Cross in the Father's judgment. And we know the extent of God's love because He sent His own Son to die on the Cross for us and raised Him up after three days so that we too may live.
This mystery is inexplicable. This grace amazing. All that every sinful and broken person out there ever wanted was love. We just want to be loved, and to love. That is the core to all our actions, twisted and sinful though they may be. But God turns tragedies into beautiful things. Look to the Cross. It's all there.