“ 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. ”
 Thomas Aquinas, Summa Theologica (1274). copy citation

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“I answer that, It is pleasant to get what one desires, as stated above (Q. 23, A. 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 (A. 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) .” source