Objection 1. It would seem that wonder is not a cause of pleasure. Because wonder is the act of one who is ignorant of the nature of something, as Damascene says. But knowledge, rather than ignorance, is a cause of pleasure. Therefore wonder is not a cause of pleasure.
Objection 2. Further, wonder is the beginning of wisdom, being as it were, the road to the search of truth, as stated in the beginning of Metaph. i, 2. But "it is more pleasant to think of what we know, than to seek what we know not," as the Philosopher says (Ethic. x, 7): since in the latter case we encounter difficulties and hindrances, in the former not; while pleasure arises from an operation which is unhindered, as stated in Ethic. vii, 12,13. Therefore wonder hinders rather than causes pleasure.
Objection 3. Further, everyone takes pleasure in what he is accustomed to: wherefore the actions of habits acquired by custom, are pleasant. But "we wonder at what is unwonted," as Augustine says (Tract. xxiv in Joan.). Therefore wonder is contrary to the cause of pleasure.
On the contrary, The Philosopher says (Rhet. i, 11) that wonder is the cause of pleasure.
I answer that, It is pleasant to get what one desires, as stated above (Question 23, Article 4): and therefore the greater the desire for the thing loved, the greater the pleasure when it is attained: indeed the very increase of desire brings with it an increase of pleasure, according as it gives rise to the hope of obtaining that which is loved, since it was stated above (3, ad 3) that desire resulting from hope is a cause of pleasure. Now wonder is a kind of desire for knowledge; a desire which comes to man when he sees an effect of which the cause either is unknown to him, or surpasses his knowledge or faculty of understanding. Consequently wonder is a cause of pleasure, in so far as it includes a hope of getting the knowledge which one desires to have. For this reason whatever is wonderful is pleasing, for instance things that are scarce. Also, representations of things, even of those which are not pleasant in themselves, give rise to pleasure; for the soul rejoices in comparing one thing with another, because comparison of one thing with another is the proper and connatural act of the reason, as the Philosopher says (Poet. iv). This again is why "it is more delightful to be delivered from great danger, because it is something wonderful," as stated in Rhetor. i, 11.
Reply to Objection 1. Wonder gives pleasure, not because it implies ignorance, but in so far as it includes the desire of learning the cause, and in so far as the wonderer learns something new, i.e. that the cause is other than he had thought it to be. [According to another reading:--that he is other than he thought himself to be.]
Reply to Objection 2. Pleasure includes two things; rest in the good, and perception of this rest. As to the former therefore, since it is more perfect to contemplate the known truth, than to seek for the unknown, the contemplation of what we know, is in itself more pleasing than the research of what we do not know. Nevertheless, as to the second, it happens that research is sometimes more pleasing accidentally, in so far as it proceeds from a greater desire: for greater desire is awakened when we are conscious of our ignorance. This is why man takes the greatest pleasure in finding or learning things for the first time.
Reply to Objection 3. It is pleasant to do what we are wont to do, inasmuch as this is connatural to us, as it were. And yet things that are of rare occurrence can be pleasant, either as regards knowledge, from the fact that we desire to know something about them, in so far as they are wonderful; or as regards action, from the fact that "the mind is more inclined by desire to act intensely in things that are new," as stated in Ethic. x, 4, since more perfect operation causes more perfect pleasure.
Source: Summa Theologica (New Advent)