Objection 1. It would seem that not every pleasure assuages every pain or sorrow. For pleasure does not assuage sorrow, save in so far as it is contrary to it: for "remedies work by contraries" (Ethic. ii, 3). But not every pleasure is contrary to every sorrow; as stated above (Question 35, Article 4). Therefore not every pleasure assuages every sorrow.
Objection 2. Further, that which causes sorrow does not assuage it. But some pleasures cause sorrow; since, as stated in Ethic. ix, 4, "the wicked man feels pain at having been pleased." Therefore not every pleasure assuages sorrow.
Objection 3. Further, Augustine says (Confess. iv, 7) that he fled from his country, where he had been wont to associate with his friend, now dead: "for so should his eyes look for him less, where they were not wont to see him." Hence we may gather that those things which united us to our dead or absent friends, become burdensome to us when we mourn their death or absence. But nothing united us more than the pleasures we enjoyed in common. Therefore these very pleasures become burdensome to us when we mourn. Therefore not every pleasure assuages every sorrow.
On the contrary, The Philosopher says (Ethic. vii, 14) that "sorrow is driven forth by pleasure, both by a contrary pleasure and by any other, provided it be intense."
I answer that, As is evident from what has been said above (Question 23, Article 4), pleasure is a kind of repose of the appetite in a suitable good; while sorrow arises from something unsuited to the appetite. Consequently in movements of the appetite pleasure is to sorrow, what, in bodies, repose is to weariness, which is due to a non-natural transmutation; for sorrow itself implies a certain weariness or ailing of the appetitive faculty. Therefore just as all repose of the body brings relief to any kind of weariness, ensuing from any non-natural cause; so every pleasure brings relief by assuaging any kind of sorrow, due to any cause whatever.
Reply to Objection 1. Although not every pleasure is specifically contrary to every sorrow, yet it is generically, as stated above (Question 35, Article 4). And consequently, on the part of the disposition of the subject, any sorrow can be assuaged by any pleasure.
Reply to Objection 2. The pleasures of wicked men are not a cause of sorrow while they are enjoyed, but afterwards: that is to say, in so far as wicked men repent of those things in which they took pleasure. This sorrow is healed by contrary pleasures.
Reply to Objection 3. When there are two causes inclining to contrary movements, each hinders the other; yet the one which is stronger and more persistent, prevails in the end. Now when a man is made sorrowful by those things in which he took pleasure in common with a deceased or absent friend, there are two causes producing contrary movements. For the thought of the friend's death or absence, inclines him to sorrow: whereas the present good inclines him to pleasure. Consequently each is modified by the other. And yet, since the perception of the present moves more strongly than the memory of the past, and since love of self is more persistent than love of another; hence it is that, in the end, the pleasure drives out the sorrow. Wherefore a little further on (Confess. iv, 8) Augustine says that his "sorrow gave way to his former pleasures."
Source: Summa Theologica (New Advent)