Objection 1. It would seem that sorrow is to be shunned more than pleasure is to be sought. For Augustine says (QQ. 83, qu. 63): "There is nobody that does not shun sorrow more than he seeks pleasure." Now that which all agree in doing, seems to be natural. Therefore it is natural and right for sorrow to be shunned more than pleasure is sought.
Objection 2. Further, the action of a contrary conduces to rapidity and intensity of movement: for "hot water freezes quicker and harder," as the Philosopher says (Meteor. i, 12). But the shunning of sorrow is due to the contrariety of the cause of sorrow; whereas the desire for pleasure does not arise from any contrariety, but rather from the suitableness of the pleasant object. Therefore sorrow is shunned more eagerly than pleasure is sought.
Objection 3. Further, the stronger the passion which a man resists according to reason, the more worthy is he of praise, and the more virtuous: since "virtue is concerned with the difficult and the good" (Ethic. ii, 3). But the brave man who resists the movement of shunning sorrow, is more virtuous than the temperate man, who resists the movement of desire for pleasure: since the Philosopher says (Rhet. ii, 4) that "the brave and the just are chiefly praised." Therefore the movement of shunning sorrow is more eager than the movement of seeking pleasure.
On the contrary, Good is stronger than evil, as Dionysius declares (Div. Nom. iv). But pleasure is desirable for the sake of the good which is its object; whereas the shunning of sorrow is on account of evil. Therefore the desire for pleasure is more eager than the shunning of sorrow.
I answer that, The desire for pleasure is of itself more eager than the shunning of sorrow. The reason of this is that the cause of pleasure is a suitable good; while the cause of pain or sorrow is an unsuitable evil. Now it happens that a certain good is suitable without any repugnance at all: but it is not possible for any evil to be so unsuitable as not to be suitable in some way. Wherefore pleasure can be entire and perfect: whereas sorrow is always partial. Therefore desire for pleasure is naturally greater than the shunning of sorrow. Another reason is because the good, which is the object of pleasure, is sought for its own sake: whereas the evil, which is the object of sorrow, is to be shunned as being a privation of good: and that which is by reason of itself is stronger than that which is by reason of something else. Moreover we find a confirmation of this in natural movements. For every natural movement is more intense in the end, when a thing approaches the term that is suitable to its nature, than at the beginning, when it leaves the term that is unsuitable to its nature: as though nature were more eager in tending to what is suitable to it, than in shunning what is unsuitable. Therefore the inclination of the appetitive power is, of itself, more eager in tending to pleasure than in shunning sorrow.
But it happens accidentally that a man shuns sorrow more eagerly than he seeks pleasure: and this for three reasons. First, on the part of the apprehension. Because, as Augustine says (De Trin. x, 12), "love is felt more keenly, when we lack that which we love." Now from the lack of what we love, sorrow results, which is caused either by the loss of some loved good, or by the presence of some contrary evil. But pleasure suffers no lack of the good loved, for it rests in possession of it. Since then love is the cause of pleasure and sorrow, the latter is more the shunned, according as love is the more keenly felt on account of that which is contrary to it. Secondly, on the part of the cause of sorrow or pain, which cause is repugnant to a good that is more loved than the good in which we take pleasure. For we love the natural well-being of the body more than the pleasure of eating: and consequently we would leave the pleasure of eating and the like, from fear of the pain occasioned by blows or other such causes, which are contrary to the well-being of the body. Thirdly, on the part of the effect: namely, in so far as sorrow hinders not only one pleasure, but all.
Reply to Objection 1. The saying of Augustine that "sorrow is shunned more than pleasure is sought" is true accidentally but not simply. And this is clear from what he says after: "Since we see that the most savage animals are deterred from the greatest pleasures by fear of pain," which pain is contrary to life which is loved above all.
Reply to Objection 2. It is not the same with movement from within and movement from without. For movement from within tends to what is suitable more than it recedes from that which is unsuitable; as we remarked above in regard to natural movement. But movement from without is intensified by the very opposition: because each thing strives in its own way to resist anything contrary to it, as aiming at its own preservation. Hence violent movement is intense at first, and slackens towards the end. Now the movement of the appetitive faculty is from within: since it tends from the soul to the object. Consequently pleasure is, of itself, more to be sought than sorrow is to be shunned. But the movement of the sensitive faculty is from without, as it were from the object of the soul. Consequently the more contrary a thing is the more it is felt. And then too, accidentally, in so far as the senses are requisite for pleasure and pain, pain is shunned more than pleasure is sought.
Reply to Objection 3. A brave man is not praised because, in accordance with reason, he is not overcome by any kind of sorrow or pain whatever, but because he is not overcome by that which is concerned with the dangers of death. And this kind of sorrow is more shunned, than pleasures of the table or of sexual intercourse are sought, which latter pleasures are the object of temperance: thus life is loved more than food and sexual pleasure. But the temperate man is praised for refraining from pleasures of touch, more than for not shunning the pains which are contrary to them, as is stated in Ethic. iii, 11.
Source: Summa Theologica (New Advent)