Objection 1. It would seem that sorrow cannot be a useful good. For it is written (Sirach 30:25): "Sadness hath killed many, and there is no profit in it."
Objection 2. Further, choice is of that which is useful to an end. But sorrow is not an object of choice; in fact, "a thing without sorrow is to be chosen rather than the same thing with sorrow" (Topic. iii, 2). Therefore sorrow is not a useful good.
Objection 3. Further, "Everything is for the sake of its own operation," as stated in De Coel. ii, 3. But "sorrow hinders operation," as stated in Ethic. x, 5. Therefore sorrow is not a useful good.
On the contrary, The wise man seeks only that which is useful. But according to Ecclesiastes 7:5, "the heart of the wise is where there is mourning, and the heart of fools where there is mirth." Therefore sorrow is useful.
I answer that, A twofold movement of the appetite ensues from a present evil. One is that whereby the appetite is opposed to the present evil; and, in this respect, sorrow is of no use; because that which is present, cannot be not present. The other movement arises in the appetite to the effect of avoiding or expelling the saddening evil: and, in this respect, sorrow is of use, if it be for something which ought to be avoided. Because there are two reasons for which it may be right to avoid a thing. First, because it should be avoided in itself, on account of its being contrary to good; for instance, sin. Wherefore sorrow for sin is useful as inducing a man to avoid sin: hence the Apostle says (2 Corinthians 7:9): "I am glad: not because you were made sorrowful, but because you were made sorrowful unto penance." Secondly, a thing is to be avoided, not as though it were evil in itself, but because it is an occasion of evil; either through one's being attached to it, and loving it too much, or through one's being thrown headlong thereby into an evil, as is evident in the case of temporal goods. And, in this respect, sorrow for temporal goods may be useful; according to Ecclesiastes 7:3: "It is better to go to the house of mourning, than to the house of feasting: for in that we are put in mind of the end of all."
Moreover, sorrow for that which ought to be avoided is always useful, since it adds another motive for avoiding it. Because the very evil is in itself a thing to be avoided: while everyone avoids sorrow for its own sake, just as everyone seeks the good, and pleasure in the good. Therefore just as pleasure in the good makes one seek the good more earnestly, so sorrow for evil makes one avoid evil more eagerly.
Reply to Objection 1. This passage is to be taken as referring to excessive sorrow, which consumes the soul: for such sorrow paralyzes the soul, and hinders it from shunning evil, as stated above (Question 37, Article 2).
Reply to Objection 2. Just as any object of choice becomes less eligible by reason of sorrow, so that which ought to be shunned is still more to be shunned by reason of sorrow: and, in this respect, sorrow is useful.
Reply to Objection 3. Sorrow caused by an action hinders that action: but sorrow for the cessation of an action, makes one do it more earnestly.
Source: Summa Theologica (New Advent)