Its emphasis is not on us (our sinfulness) but on God’s mercy and salvific action in Jesus Christ. It could just as accurately be translated "O Lord, you are merciful!" Note that the sample tropes all mention what Christ has done for us, not how we have sinned. For example, “you were sent to heal the contrite,” “you have shown us the way to the Father,” or “you come in word and sacrament to strengthen us in holiness,” leading to further acclamation of God’s praises in the Gloria.