St. Thomas Aquinas
“Such is the dignity of this sacrament that it is performed only as in the person of Christ. Now whoever performs any act in another’s stead, must do so by the power bestowed by such a one. But as the power of receiving this sacrament is conceded by Christ to the baptized person, so likewise the power of consecrating this sacrament on Christ’s behalf is bestowed upon the priest at his ordination: for thereby he is put upon a level with them to whom the Lord said (Lk. 22:19): ‘Do this for a commemoration of Me.’ Therefore, it must be said that it belongs to priests to accomplish this sacrament.”
Source: St. Thomas Aquinas, Summa Theologica