Objection 1. It would seem that it is not necessary to confess to a priest. For we are not bound to confession, except in virtue of its Divine institution. Now its Divine institution is made known to us (James 5:16): "Confess your sins, one to another," where there is no mention of a priest. Therefore it is not necessary to confess to a priest.
Objection 2. Further, Penance is a necessary sacrament, as is also Baptism. But any man is the minister of Baptism, on account of its necessity. Therefore any man is the minister of Penance. Now confession should be made to the minister of Penance. Therefore it suffices to confess to anyone.
Objection 3. Further, confession is necessary in order that the measure of satisfaction should be imposed on the penitent. Now, sometimes another than a priest might be more discreet than many priests are in imposing the measure of satisfaction on the penitent. Therefore it is not necessary to confess to a priest.
Objection 4. Further, confession was instituted in the Church in order that the rectors might know their sheep by sight. But sometimes a rector or prelate is not a priest. Therefore confession should not always be made to a priest.
On the contrary, The absolution of the penitent, for the sake of which he makes his confession, is imparted by none but priests to whom the keys are intrusted. Therefore confession should be made to a priest.
Further, confession is foreshadowed in the raising of the dead Lazarus to life. Now our Lord commanded none but the disciples to loose Lazarus (John 11:44). Therefore confession should be made to a priest.
I answer that, The grace which is given in the sacraments, descends from the Head to the members. Wherefore he alone who exercises a ministry over Christ's true body is a minister of the sacraments, wherein grace is given; and this belongs to a priest alone, who can consecrate the Eucharist. Therefore, since grace is given in the sacrament of Penance, none but a priest is the minister of the sacrament: and consequently sacramental confession which should be made to a minister of the Church, should be made to none but a priest.
Reply to Objection 1. James speaks on the presupposition of the Divine institutions: and since confession had already been prescribed by God to be made to a priest, in that He empowered them, in the person of the apostles, to forgive sins, as related in John 20:23, we must take the words of James as conveying an admonishment to confess to priests.
Reply to Objection 2. Baptism is a sacrament of greater necessity than Penance, as regards confession and absolution, because sometimes Baptism cannot be omitted without loss of eternal salvation, as in the case of children who have not come to the use of reason: whereas this cannot be said of confession and absolution, which regard none but adults, in whom contrition, together with the purpose of confessing and the desire of absolution, suffices to deliver them from everlasting death. Consequently there is no parity between Baptism and confession.
Reply to Objection 3. In satisfaction we must consider not only the quantity of the punishment but also its power, inasmuch as it is part of a sacrament. In this way it requires a dispenser of the sacraments, though the quantity of the punishment may be fixed by another than a priest.
Reply to Objection 4. It may be necessary for two reasons to know the sheep by sight. First, in order to register them as members of Christ's flock, and to know the sheep by sight thus belongs to the pastoral charge and care, which is sometimes the duty of those who are not priests. Secondly, that they may be provided with suitable remedies for their health; and to know the sheep by sight thus belongs to the man, i.e. the priest, whose business it is to provide remedies conducive to health, such as the sacrament of the Eucharist, and other like things. It is to this knowledge of the sheep that confession is ordained.
Source: Summa Theologica (New Advent)