35 And surely every reason We have urged in showing the dignity of the Catholic priesthood does but reinforce its obligation of singular holiness; for as the Angelic Doctor teaches: "To fulfill the duties of Holy Orders, common goodness does not suffice; but excelling goodness is required; that they who receive Orders and are thereby higher in rank than the people, may also be higher in holiness." The Eucharistic Sacrifice in which the Immaculate Victim who taketh away the sins of the world is immolated, requires in a special way that the priest, by a holy and spotless life, should make himself as far as he can, less unworthy of God, to whom he daily offers that adorable Victim, the very Word of God incarnate for love of us. Agnoscite quod agitis, imitamini quod tractatis , "realize what you are doing, and imitate what you handle," says the Church through the Bishop to the deacons as they are about to be consecrated priests. The priest is also the almoner of God's graces of which the Sacraments are the channels; how grave a reproach would it be, for one who dispenses these most precious graces were he himself without them, or were he even to esteem them lightly and guard them with little care.
Source: Ad Catholici Sacerdotii (Vatican.va)