Objection 1. It would seem that the imposition of the priest's hands is necessary for this sacrament. For it is written (Mark 16:18): "They shall lay hands upon the sick, and they shall recover." Now sinners are sick spiritually, and obtain recovery through this sacrament. Therefore an imposition of hands should be made in this sacrament.
Objection 2. Further, in this sacrament man regains the Holy Ghost Whom he had lost, wherefore it is said in the person of the penitent (Psalm 1:14): "Restore unto me the joy of Thy salvation, and strengthen me with a perfect spirit." Now the Holy Ghost is given by the imposition of hands; for we read (Acts 8:17) that the apostles "laid their hands upon them, and they received the Holy Ghost"; and (Matthew 19:13) that "little children were presented" to our Lord, "that He should impose hands upon them." Therefore an imposition of hands should be made in this sacrament.
Objection 3. Further, the priest's words are not more efficacious in this than in the other sacraments. But in the other sacraments the words of the minister do not suffice, unless he perform some action: thus, in Baptism, the priest while saying: "I baptize thee," has to perform a bodily washing. Therefore, also while saying: "I absolve thee," the priest should perform some action in regard to the penitent, by laying hands on him.
On the contrary, When our Lord said to Peter (Matthew 16:19): "Whatsoever thou shalt loose on earth," etc., He made no mention of an imposition of hands; nor did He when He said to all the apostles (John 20:13): "Whose sins you shall forgive, they are forgiven them." Therefore no imposition of hands is required for this sacrament.
I answer that, In the sacraments of the Church the imposition of hands is made, to signify some abundant effect of grace, through those on whom the hands are laid being, as it were, united to the ministers in whom grace should be plentiful. Wherefore an imposition of hands is made in the sacrament of Confirmation, wherein the fulness of the Holy Ghost is conferred; and in the sacrament of order, wherein is bestowed a certain excellence of power over the Divine mysteries; hence it is written (2 Timothy 1:6): "Stir up the grace of God which is in thee, by the imposition of my hands."
Now the sacrament of Penance is ordained, not that man may receive some abundance of grace, but that his sins may be taken away; and therefore no imposition of hands is required for this sacrament, as neither is there for Baptism, wherein nevertheless a fuller remission of sins is bestowed.
Reply to Objection 1. That imposition of hands is not sacramental, but is intended for the working of miracles, namely, that by the contact of a sanctified man's hand, even bodily infirmity might be removed; even as we read of our Lord (Mark 6:5) that He cured the sick, "laying His hands upon them," and (Matthew 8:3) that He cleansed a leper by touching him.
Reply to Objection 2. It is not every reception of the Holy Ghost that requires an imposition of hands, since even in Baptism man receives the Holy Ghost, without any imposition of hands: it is at the reception of the fulness of the Holy Ghost which belongs to Confirmation that an imposition of hands is required.
Reply to Objection 3. In those sacraments which are perfected in the use of the matter, the minister has to perform some bodily action on the recipient of the sacrament, e.g. in Baptism, Confirmation, and Extreme Unction; whereas this sacrament does not consist in the use of matter employed outwardly, the matter being supplied by the part taken by the penitent: wherefore, just as in the Eucharist the priest perfects the sacrament by merely pronouncing the words over the matter, so the mere words which the priest while absolving pronounces over the penitent perfect the sacrament of absolution. If, indeed, any bodily act were necessary on the part of the priest, the sign of the cross, which is employed in the Eucharist, would not be less becoming than the imposition of hands, in token that sins are forgiven through the blood of Christ crucified; and yet this is not essential to this sacrament as neither is it to the Eucharist.
Source: Summa Theologica (New Advent)