Youcat commented through CCC – Question n. 236 - Part I.

YOUCAT Question n. 236 - Part I. Why are priests the only ones who can forgive sins?

(Youcat answer) No man can forgive sins unless he has a commission from God to do so and the power given by him to ensure that the forgiveness he promises the penitent really takes place. The Bishop, in the first place, is appointed to do that and, then, his helpers, the ordained priests.

A deepening through CCC

(CCC 1461) Since Christ entrusted to his apostles the ministry of reconciliation (Cf. Jn 20:23; 2 Cor 5:18), bishops who are their successors, and priests, the bishops' collaborators, continue to exercise this ministry. Indeed bishops and priests, by virtue of the sacrament of Holy Orders, have the power to forgive all sins "in the name of the Father, and of the Son, and of the Holy Spirit." 

Reflecting and meditating 

(CCC Comment)

(CCC 1462) Forgiveness of sins brings reconciliation with God, but also with the Church. Since ancient times the bishop, visible head of a particular Church, has thus rightfully been considered to be the one who principally has the power and ministry of reconciliation: he is the moderator of the penitential discipline (Cf. LG 26 § 3). Priests, his collaborators, exercise it to the extent that they have received the commission either from their bishop (or religious superior) or the Pope, according to the law of the Church (Cf. CIC cann. 844; 967-969; 972; CCEO, can. 722 §§ 3-4). 

(This question: Why are priests the only ones who can forgive sins? is continued)