Objection 1. It would seem that it is not expedient to grieve for sin continually. For it is sometimes expedient to rejoice, as is evident from Philippians 4:4, where the gloss on the words, "Rejoice in the Lord always," says that "it is necessary to rejoice." Now it is not possible to rejoice and grieve at the same time. Therefore it is not expedient to grieve for sin continually.
Objection 2. Further, that which, in itself, is an evil and a thing to be avoided should not be taken upon oneself, except in so far as it is necessary as a remedy against something, as in the case of burning or cutting a wound. Now sorrow is in itself an evil; wherefore it is written (Sirach 30:24): "Drive away sadness far from thee," and the reason is given (Sirach 30:25): "For sadness hath killed many, and there is no profit in it." Moreover the Philosopher says the same (Ethic. vii, 13,14; x, 5). Therefore one should not grieve for sin any longer than suffices for the sin to be blotted out. Now sin is already blotted out after the first sorrow of contrition. Therefore it is not expedient to grieve any longer.
Objection 3. Further, Bernard says (Serm. xi in Cant.): "Sorrow is a good thing, if it is not continual; for honey should be mingled with wormwood." Therefore it seems that it is inexpedient to grieve continually.
On the contrary, Augustine [De vera et falsa Poenitentia, work of an unknown author] says: "The penitent should always grieve, and rejoice in his grief."
Further, it is expedient always to continue, as far as it is possible, those acts in which beatitude consists. Now such is sorrow for sin, as is shown by the words of Matthew 5:5, "Blessed are they that mourn." Therefore it is expedient for sorrow to be as continual as possible.
I answer that, We find this condition in the acts of the virtues, that in them excess and defect are not possible, as is proved in Ethic. ii, 6,7. Wherefore, since contrition, so far as it is a kind of displeasure seated in the rational appetite, is an act of the virtue of penance, there can never be excess in it, either as to its intensity, or as to its duration, except in so far as the act of one virtue hinders the act of another which is more urgent for the time being. Consequently the more continually a man can perform acts of this displeasure, the better it is, provided he exercises the acts of other virtues when and how he ought to. On the other hand, passions can have excess and defect, both in intensity and in duration. Wherefore, as the passion of sorrow, which the will takes upon itself, ought to be moderately intense, so ought it to be of moderate duration, lest, if it should last too long, man fall into despair, cowardice, and such like vices.
Reply to Objection 1. The sorrow of contrition is a hindrance to worldly joy, but not to the joy which is about God, and which has sorrow itself for object.
Reply to Objection 2. The words of Ecclesiasticus refer to worldly joy: and the Philosopher is referring to sorrow as a passion, of which we should make moderate use, according as the end, for which it is assumed, demands.
Reply to Objection 3. Bernard is speaking of sorrow as a passion.
Source: Summa Theologica (New Advent)