Objection 1. It would seem that likeness is not a cause of pleasure. Because ruling and presiding seem to imply a certain unlikeness. But "it is natural to take pleasure in ruling and presiding," as stated in Rhetor. i, 11. Therefore unlikeness, rather than likeness, is a cause of pleasure.
Objection 2. Further, nothing is more unlike pleasure than sorrow. But those who are burdened by sorrow are most inclined to seek pleasures, as the Philosopher says (Ethic. vii, 14). Therefore unlikeness, rather than likeness, is a cause of pleasure.
Objection 3. Further, those who are satiated with certain delights, derive not pleasure but disgust from them; as when one is satiated with food. Therefore likeness is not a cause of pleasure.
On the contrary, Likeness is a cause of love, as above stated (27, 3): and love is the cause of pleasure. Therefore likeness is a cause of pleasure.
I answer that, Likeness is a kind of unity; hence that which is like us, as being one with us, causes pleasure; just at it causes love, as stated above (Question 27, Article 3). And if that which is like us does not hurt our own good, but increase it, it is pleasurable simply; for instance one man in respect of another, one youth in relation to another. But if it be hurtful to our own good, thus accidentally it causes disgust or sadness, not as being like and one with us, but as hurtful to that which is yet more one with us.
Now it happens in two ways that something like is hurtful to our own good. First, by destroying the measure of our own good, by a kind of excess; because good, especially bodily good, as health, is conditioned by a certain measure: wherefore superfluous good or any bodily pleasure, causes disgust. Secondly, by being directly contrary to one's own good: thus a potter dislikes other potters, not because they are potters, but because they deprive him of his own excellence or profits, which he seeks as his own good.
Reply to Objection 1. Since ruler and subject are in communion with one another, there is a certain likeness between them: but this likeness is conditioned by a certain superiority, since ruling and presiding pertain to the excellence of a man's own good: because they belong to men who are wise and better than others; the result being that they give man an idea of his own excellence. Another reason is that by ruling and presiding, a man does good to others, which is pleasant.
Reply to Objection 2. That which gives pleasure to the sorrowful man, though it be unlike sorrow, bears some likeness to the man that is sorrowful: because sorrows are contrary to his own good. Wherefore the sorrowful man seeks pleasure as making for his own good, in so far as it is a remedy for its contrary. And this is why bodily pleasures, which are contrary to certain sorrows, are more sought than intellectual pleasures, which have no contrary sorrow, as we shall state later on (35, 5). And this explains why all animals naturally desire pleasure: because animals ever work through sense and movement. For this reason also young people are most inclined to seek pleasures; on account of the many changes to which they are subject, while yet growing. Moreover this is why the melancholic has a strong desire for pleasures, in order to drive away sorrow: because his "body is corroded by a base humor," as stated in Ethic. vii, 14.
Reply to Objection 3. Bodily goods are conditioned by a certain fixed measure: wherefore surfeit of such things destroys the proper good, and consequently gives rise to disgust and sorrow, through being contrary to the proper good of man.
Source: Summa Theologica (New Advent)