Objection 1. It would seem that not all are bound to confession, for Jerome says on Isaiah 3:9 ("They have proclaimed abroad"), "their sin," etc.: "Penance is the second plank after shipwreck." But some have not suffered shipwreck after Baptism. Therefore Penance is not befitting them, and consequently neither is confession which is a part of Penance.
Objection 2. Further, it is to the judge that confession should be made in any court. But some have no judge over them. Therefore they are not bound to confession.
Objection 3. Further, some have none but venial sins. Now a man is not bound to confess such sins. Therefore not everyone is bound to confession.
On the contrary, Confession is condivided with satisfaction and contrition. Now all are bound to contrition and satisfaction. Therefore all are bound to confession also.
Further, this appears from the Decretals (De Poenit. et Remiss. xii), where it is stated that "all of either sex are bound to confess their sins as soon as they shall come to the age of discretion."
I answer that, We are bound to confession on two counts: first, by the Divine law, from the very fact that confession is a remedy, and in this way not all are bound to confession, but those only who fall into mortal sin after Baptism; secondly, by a precept of positive law, and in this way all are bound by the precept of the Church laid down in the general council (Lateran IV, Can. 21) under Innocent III, both in order that everyone may acknowledge himself to be a sinner, because "all have sinned and need the grace of God" (Romans 3:23); and that the Eucharist may be approached with greater reverence; and lastly, that parish priests may know their flock, lest a wolf may hide therein.
Reply to Objection 1. Although it is possible for a man, in this mortal life, to avoid shipwreck, i.e. mortal sin, after Baptism, yet he cannot avoid venial sins, which dispose him to shipwreck, and against which also Penance is ordained; wherefore there is still room for Penance, and consequently for confession, even in those who do not commit mortal sins.
Reply to Objection 2. All must acknowledge Christ as their judge, to Whom they must confess in the person of His vicar; and although the latter may be the inferior if the penitent be a prelate, yet he is the superior, in so far as the penitent is a sinner, while the confessor is the minister of Christ.
Reply to Objection 3. A man is bound to confess his venial sins, not in virtue of the sacrament, but by the institution of the Church, and that, when he has no other sins to confess. We may also, with others, answer that the Decretal quoted above does not bind others than those who have mortal sins to confess. This is evident from the fact that it orders all sins to be confessed, which cannot apply to venial sins, because no one can confess all his venial sins. Accordingly, a man who has no mortal sins to confess, is not bound to confess his venial sins, but it suffices for the fulfillment of the commandment of the Church that he present himself before the priest, and declare himself to be unconscious of any mortal sin: and this will count for his confession.
Source: Summa Theologica (New Advent)