Objection 1. It would seem that delight is not a passion. For Damascene (De Fide Orth. ii, 22) distinguishes operation from passion, and says that "operation is a movement in accord with nature, while passion is a movement contrary to nature." But delight is an operation, according to the Philosopher (Ethic. vii, 12; x, 5). Therefore delight is not a passion.
Objection 2. Further, "To be passive is to be moved," as stated in Phys. iii, 3. But delight does not consist in being moved, but in having been moved; for it arises from good already gained. Therefore delight is not a passion.
Objection 3. Further, delight is a kind of a perfection of the one who is delighted; since it "perfects operation," as stated in Ethic. x, 4,5. But to be perfected does not consist in being passive or in being altered, as stated in Phys. vii, 3 and De Anima ii, 5. Therefore delight is not a passion.
On the contrary, Augustine (De Civ. Dei ix, 2; xiv, 5 seqq) reckons delight, joy, or gladness among the other passions of the soul.
I answer that, The movements of the sensitive appetite, are properly called passions, as stated above (Question 22, Article 3). Now every emotion arising from a sensitive apprehension, is a movement of the sensitive appetite: and this must needs be said of delight, since, according to the Philosopher (Rhet. i, 11) "delight is a certain movement of the soul and a sensible establishing thereof all at once, in keeping with the nature of the thing."
In order to understand this, we must observe that just as in natural things some happen to attain to their natural perfections, so does this happen in animals. And though movement towards perfection does not occur all at once, yet the attainment of natural perfection does occur all at once. Now there is this difference between animals and other natural things, that when these latter are established in the state becoming their nature, they do not perceive it, whereas animals do. And from this perception there arises a certain movement of the soul in the sensitive appetite; which movement is called delight. Accordingly by saying that delight is "a movement of the soul," we designate its genus. By saying that it is "an establishing in keeping with the thing's nature," i.e. with that which exists in the thing, we assign the cause of delight, viz. the presence of a becoming good. By saying that this establishing is "all at once," we mean that this establishing is to be understood not as in the process of establishment, but as in the fact of complete establishment, in the term of the movement, as it were: for delight is not a "becoming" as Plato [Phileb. 32,33 maintained, but a "complete fact," as stated in Ethic. vii, 12. Lastly, by saying that this establishing is "sensible," we exclude the perfections of insensible things wherein there is no delight. It is therefore evident that, since delight is a movement of the animal appetite arising from an apprehension of sense, it is a passion of the soul.
Reply to Objection 1. Connatural operation, which is unhindered, is a second perfection, as stated in De Anima ii, 1: and therefore when a thing is established in its proper connatural and unhindered operation, delight follows, which consists in a state of completion, as observed above. Accordingly when we say that delight is an operation, we designate, not its essence, but its cause.
Reply to Objection 2. A twofold movement is to be observed in an animal: one, according to the intention of the end, and this belongs to the appetite; the other, according to the execution, and this belongs to the external operation. And so, although in him who has already gained the good in which he delights, the movement of execution ceases, by which the tends to the end; yet the movement of the appetitive faculty does not cease, since, just as before it desired that which it had not, so afterwards does it delight in that which is possesses. For though delight is a certain repose of the appetite, if we consider the presence of the pleasurable good that satisfies the appetite, nevertheless there remains the impression made on the appetite by its object, by reason of which delight is a kind of movement.
Reply to Objection 3. Although the name of passion is more appropriate to those passions which have a corruptive and evil tendency, such as bodily ailments, as also sadness and fear in the soul; yet some passions have a tendency to something good, as stated above (23, 1,4): and in this sense delight is called a passion.
Source: Summa Theologica (New Advent)