7 Now though in both these matters we are impelled by quite the same motives, none the less we are holden to the duty of reparation and expiation by a certain more valid title of justice and of love, of justice indeed, in order that the offense offered to God by our sins may be expiated and that the violated order may be repaired by penance: and of love too so that we may suffer together with Christ suffering and "filled with reproaches" ( Lam . iii, 30), and for all our poverty may offer Him some little solace. For since we are all sinners and laden with many faults, our God must be honored by us not only by that worship wherewith we adore His infinite Majesty with due homage, or acknowledge His supreme dominion by praying, or praise His boundless bounty by thanksgiving; but besides this we must need make satisfaction to God the just avenger, "for our numberless sins and offenses and negligences." To Consecration, therefore, whereby we are devoted to God and are called holy to God, by that holiness and stability which, as the Angelic Doctor teaches, is proper to consecration (2da. 2dae. qu. 81, a. 8. c.), there must be added expiation, whereby sins are wholly blotted out, lest the holiness of the supreme justice may punish our shameless unworthiness, and reject our offering as hateful rather than accept it as pleasing.
Source: Miserentissimus Redemptor (Vatican.va)