Pleasure.

All men take a natural pleasure in learning quickly words which denote something; and so those words are pleasantest which give us new knowledge. Strange words have no meaning for us; common terms we know already; it is metaphor which gives us most of this pleasure. Thus, when the poet calls old age "a dried stalk," he gives us a new perception by means of the common genus; for both the things have lost their bloom. A simile, as has been said before, is a metaphor with a preface; for this reason it is less pleasing because it is more lengthy; nor does it affirm that this is that; and so the mind does not even inquire into the matter. It follows that a smart style, and a smart enthymeme, are those which give us a new and rapid perception.

Aristotle, from Rhetoric