Objection 1. It would seem that it is not an effect of sorrow to burden the soul. For the Apostle says (2 Corinthians 7:11): "Behold this self-same thing, that you were made sorrowful according to God, how great carefulness it worketh in you: yea, defence, yea indignation," etc. Now carefulness and indignation imply that the soul is uplifted, which is contrary to being depressed. Therefore depression is not an effect of sorrow.
Objection 2. Further, sorrow is contrary to pleasure. But the effect of pleasure is expansion: the opposite of which is not depression but contraction. Therefore depression should not be reckoned as an effect of sorrow.
Objection 3. Further, sorrow consumes those who are inflicted therewith, as may be gathered from the words of the Apostle (2 Corinthians 2:7): "Lest perhaps such an one be swallowed up with overmuch sorrow." But that which is depressed is not consumed; nay, it is weighed down by something heavy, whereas that which is consumed enters within the consumer. Therefore depression should not be reckoned an effect of sorrow.
On the contrary, Gregory of Nyssa [Nemesius, De Nat. Hom. xix.] and Damascene (De Fide Orth. ii, 14) speak of "depressing sorrow."
I answer that, The effects of the soul's passions are sometimes named metaphorically, from a likeness to sensible bodies: for the reason that the movements of the animal appetite are like the inclinations of the natural appetite. And in this way fervor is ascribed to love, expansion to pleasure, and depression to sorrow. For a man is said to be depressed, through being hindered in his own movement by some weight. Now it is evident from what has been said above (23, 4; 25, 4; 36, 01) that sorrow is caused by a present evil: and this evil, from the very fact that it is repugnant to the movement of the will, depresses the soul, inasmuch as it hinders it from enjoying that which it wishes to enjoy. And if the evil which is the cause of sorrow be not so strong as to deprive one of the hope of avoiding it, although the soul be depressed in so far as, for the present, it fails to grasp that which it craves for; yet it retains the movement whereby to repulse that evil. If, on the other hand, the strength of the evil be such as to exclude the hope of evasion, then even the interior movement of the afflicted soul is absolutely hindered, so that it cannot turn aside either this way or that. Sometimes even the external movement of the body is paralyzed, so that a man becomes completely stupefied.
Reply to Objection 1. That uplifting of the soul ensues from the sorrow which is according to God, because it brings with it the hope of the forgiveness of sin.
Reply to Objection 2. As far as the movement of the appetite is concerned, contraction and depression amount to the same: because the soul, through being depressed so as to be unable to attend freely to outward things, withdraws to itself, closing itself up as it were.
Reply to Objection 3. Sorrow is said to consume man, when the force of the afflicting evil is such as to shut out all hope of evasion: and thus also it both depresses and consumes at the same time. For certain things, taken metaphorically, imply one another, which taken literally, appear to exclude one another.
Source: Summa Theologica (New Advent)