Objection 1. It would seem that operation is not the proper and first cause of pleasure. For, as the Philosopher says (Rhet. i, 11), "pleasure consists in a perception of the senses," since knowledge is requisite for pleasure, as stated above (Question 31, Article 1). But the objects of operations are knowable before the operations themselves. Therefore operation is not the proper cause of pleasure.
Objection 2. Further, pleasure consists especially in an end gained: since it is this that is chiefly desired. But the end is not always an operation, but is sometimes the effect of the operation. Therefore operation is not the proper and direct cause of pleasure.
Objection 3. Further, leisure and rest consist in cessation from work: and they are objects of pleasure (Rhet. i, 11). Therefore operation is not the proper cause of pleasure.
On the contrary, The Philosopher says (Ethic. vii, 12,13; x, 4) that "pleasure is a connatural and uninterrupted operation."
I answer that, As stated above (Question 31, Article 1), two things are requisite for pleasure: namely, the attainment of the suitable good, and knowledge of this attainment. Now each of these consists in a kind of operation: because actual knowledge is an operation; and the attainment of the suitable good is by means of an operation. Moreover, the proper operation itself is a suitable good. Wherefore every pleasure must needs be the result of some operation.
Reply to Objection 1. The objects of operations are not pleasurable save inasmuch as they are united to us; either by knowledge alone, as when we take pleasure in thinking of or looking at certain things; or in some other way in addition to knowledge; as when a man takes pleasure in knowing that he has something good--riches, honor, or the like; which would not be pleasurable unless they were apprehended as possessed. For as the Philosopher observes (Polit. ii, 2) "we take great pleasure in looking upon a thing as our own, by reason of the natural love we have for ourselves." Now to have such like things is nothing else but to use them or to be able to use them: and this is through some operation. Wherefore it is evident that every pleasure is traced to some operation as its cause.
Reply to Objection 2. Even when it is not an operation, but the effect of an operation, that is the end, this effect is pleasant in so far as possessed or effected: and this implies use or operation.
Reply to Objection 3. Operations are pleasant, in so far as they are proportionate and connatural to the agent. Now, since human power is finite, operation is proportionate thereto according to a certain measure. Wherefore if it exceed that measure, it will be no longer proportionate or pleasant, but, on the contrary, painful and irksome. And in this sense, leisure and play and other things pertaining to repose, are pleasant, inasmuch as they banish sadness which results from labor.
Source: Summa Theologica (New Advent)