Objection 1. It would seem that sorrow is caused by the loss of a good rather than by the presence of an evil. For Augustine says (De viii QQ. Dulcit. qu. 1) that sorrow is caused by the loss of temporal goods. Therefore, in like manner, every sorrow is caused by the loss of some good.
Objection 2. Further, it was said above (Question 35, Article 4) that the sorrow which is contrary to a pleasure, has the same object as that pleasure. But the object of pleasure is good, as stated above (23, 4; 31, 1; 35, 3). Therefore sorrow is caused chiefly by the loss of good.
Objection 3. Further, according to Augustine (De Civ. Dei xiv, 7,9), love is the cause of sorrow, as of the other emotions of the soul. But the object of love is good. Therefore pain or sorrow is felt for the loss of good rather than for an evil that is present.
On the contrary, Damascene says (De Fide Orth. ii, 12) that "the dreaded evil gives rise to fear, the present evil is the cause of sorrow."
I answer that, If privations, as considered by the mind, were what they are in reality, this question would seem to be of no importance. For, as stated in the I, 14, 10 and I, 48, 3, evil is the privation of good: and privation is in reality nothing else than the lack of the contrary habit; so that, in this respect, to sorrow for the loss of good, would be the same as to sorrow for the presence of evil. But sorrow is a movement of the appetite in consequence of an apprehension: and even a privation, as apprehended, has the aspect of a being, wherefore it is called "a being of reason." And in this way evil, being a privation, is regarded as a "contrary." Accordingly, so far as the movement of the appetite is concerned, it makes a difference which of the two it regards chiefly, the present evil or the good which is lost.
Again, since the movement of the animal appetite holds the same place in the actions of the soul, as natural movement in natural things; the truth of the matter is to be found by considering natural movements. For if, in natural movements, we observe those of approach and withdrawal, approach is of itself directed to something suitable to nature; while withdrawal is of itself directed to something contrary to nature; thus a heavy body, of itself, withdraws from a higher place, and approaches naturally to a lower place. But if we consider the cause of both these movements, viz. gravity, then gravity itself inclines towards the lower place more than it withdraws from the higher place, since withdrawal from the latter is the reason for its downward tendency.
Accordingly, since, in the movements of the appetite, sorrow is a kind of flight or withdrawal, while pleasure is a kind of pursuit or approach; just as pleasure regards first the good possessed, as its proper object, so sorrow regards the evil that is present. On the other hand love, which is the cause of pleasure and sorrow, regards good rather than evil: and therefore, forasmuch as the object is the cause of a passion, the present evil is more properly the cause of sorrow or pain, than the good which is lost.
Reply to Objection 1. The loss itself of good is apprehended as an evil, just as the loss of evil is apprehended as a good: and in this sense Augustine says that pain results from the loss of temporal goods.
Reply to Objection 2. Pleasure and its contrary pain have the same object, but under contrary aspects: because if the presence of a particular thin be the object of pleasure, the absence of that same thing is the object of sorrow. Now one contrary includes the privation of the other, as stated in Metaph. x, 4: and consequently sorrow in respect of one contrary is, in a way, directed to the same thing under a contrary aspect.
Reply to Objection 3. When many movements arise from one cause, it does not follow that they all regard chiefly that which the cause regards chiefly, but only the first of them. And each of the others regards chiefly that which is suitable to it according to its own nature.
Source: Summa Theologica (New Advent)