Objection 1. It would seem that doing good to another is not a cause of pleasure. Because pleasure is caused by one's obtaining one's proper good, as stated above (1,5; 31, 1). But doing good pertains not to the obtaining but to the spending of one's proper good. Therefore it seems to be the cause of sadness rather than of pleasure.
Objection 2. Further, the Philosopher says (Ethic. iv, 1) that "illiberality is more connatural to man than prodigality." Now it is a mark of prodigality to do good to others; while it is a mark of illiberality to desist from doing good. Since therefore everyone takes pleasure in a connatural operation, as stated in Ethic. vii, 14 and x, 4, it seems that doing good to others is not a cause of pleasure.
Objection 3. Further, contrary effects proceed from contrary causes. But man takes a natural pleasure in certain kinds of ill-doing, such as overcoming, contradicting or scolding others, or, if he be angry, in punishing them, as the Philosopher says (Rhet. i, 11). Therefore doing good to others is a cause of sadness rather than pleasure.
On the contrary, The Philosopher says (Polit. ii, 2) that "it is most pleasant to give presents or assistance to friends and strangers."
I answer that, Doing good to another may give pleasure in three ways. First, in consideration of the effect, which is the good conferred on another. In this respect, inasmuch as through being united to others by love, we look upon their good as being our own, we take pleasure in the good we do to others, especially to our friends, as in our own good. Secondly, in consideration of the end; as when a man, from doing good to another, hopes to get some good for himself, either from God or from man: for hope is a cause of pleasure. Thirdly, in consideration of the principle: and thus, doing good to another, can give pleasure in respect of a threefold principle. One is the faculty of doing good: and in this regard, doing good to another becomes pleasant, in so far as it arouses in man an imagination of abundant good existing in him, whereof he is able to give others a share. Wherefore men take pleasure in their children, and in their own works, as being things on which they bestow a share of their own good. Another principle is man's habitual inclination to do good, by reason of which doing good becomes connatural to him: for which reason the liberal man takes pleasure in giving to others. The third principle is the motive: for instance when a man is moved by one whom he loves, to do good to someone: for whatever we do or suffer for a friend is pleasant, because love is the principal cause of pleasure.
Reply to Objection 1. Spending gives pleasure as showing forth one's good. But in so far as it empties us of our own good it may be a cause of sadness; for instance when it is excessive.
Reply to Objection 2. Prodigality is an excessive spending, which is unnatural: wherefore prodigality is said to be contrary to nature.
Reply to Objection 3. To overcome, to contradict, and to punish, give pleasure, not as tending to another's ill, but as pertaining to one's own good, which man loves more than he hates another's ill. For it is naturally pleasant to overcome, inasmuch as it makes a man to appreciate his own superiority. Wherefore all those games in which there is a striving for the mastery, and a possibility of winning it, afford the greatest pleasure: and speaking generally all contests, in so far as they admit hope of victory. To contradict and to scold can give pleasure in two ways. First, as making man imagine himself to be wise and excellent; since it belongs to wise men and elders to reprove and to scold. Secondly, in so far as by scolding and reproving, one does good to another: for this gives one pleasure, as stated above. It is pleasant to an angry man to punish, in so far as he thinks himself to be removing an apparent slight, which seems to be due to a previous hurt: for when a man is hurt by another, he seems to be slighted thereby; and therefore he wishes to be quit of this slight by paying back the hurt. And thus it is clear that doing good to another may be of itself pleasant: whereas doing evil to another is not pleasant, except in so far as it seems to affect one's own good.
Source: Summa Theologica (New Advent)