God chooses us, we don’t choose him. Grace is a gift that we can reject. When we sin we tell God we don’t want his grace, when we go to the sacrament, we ask for it back. God, being always forgiving forgets our wrongdoing and fills us with grace again.
From Pillar of Fire Pillar of Truth:
Penance (CCC 1422–1498)
Sometimes on our journey toward the heavenly promised land we stumble and fall into sin. God is always ready to lift us up and to restore us to grace-filled fellowship with him. He does this through the sacrament of penance (which is also known as confession or reconciliation).
Jesus gave his apostles power and authority to reconcile us to the Father. They received Jesus’ own power to forgive sins when he breathed on them and said, “Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained” (John 20:22–23).
Paul notes that “all this is from God, who has reconciled us to himself through Christ and given us the ministry of reconciliation. . . . So, we are ambassadors for Christ, as if God were appealing through us” (2 Cor. 5:18–20). Through confession to a priest, God’s minister, we have our sins forgiven, and we receive grace to help us resist future temptations.