Symbolism, Imagery, Allegory
Poetry and art are two areas that we definitely can't reduce to pure reason. The whole aim of these things is to provoke some kind of emotional response—this is what's often used as the main criteria when judging if they're successful. But emotional responses aren't always positive. So what does Hume sees as creating a pleasing/not so pleasing response?
Hume sees pastoral poetry as most agreeable because it's all about gentle, peaceful imagery and doesn't conjure up dramatic or painful emotions like some other varieties. In both art and life, this kind of imagery is appealing (check out an example here). Hume explains:
The eye is pleased with the prospect of corn-fields and loaded vine-yards; horses grazing, and flocks pasturing: but flies the view of briars and brambles, affording shelter to wolves and serpents. (II.II.4)
Hume suggests that the Italian poet Jacapo Sannazzaro made a wrong step in using a seashore as the setting for a pastoral scene. Hume certainly recognizes that the sea is majestic, but he feels that this was a blunder. Why? Because, rather than peaceful imagery, the poet depicts fishermen who spend their days working themselves to the bone and being in constant danger. Because we have a natural sense of sympathy, this is upsetting to us.
Far better in Hume's opinion are images such as the Elysian fields or Arcadia; again, because these present us with utopias of tranquility, love, and friendship.
Hume also contrasts the poetical fiction of the Golden Age with philosophical fiction describing humanity's natural state. Where the philosophers imagined that we started out in "a state of mutual war and violence," the poetic writers represented this era as "the most charming and most peaceable condition" (III.I.15). Hume's doubtful whether either of these extremes represents reality, but, of the two, the poetic version's definitely the one we'd go for.
Though he contrasts agreeable and disagreeable emotions, Hume recognizes that the ability to create an emotional response is a plus point in itself. Whether we're reading a book or watching a movie, the goal is to feel a sense of connection with the characters and get involved in what's going on. If we feel completely detached then we have no emotional investment and will probably start zoning out. As Hume summarizes:
It is the business of poetry to bring every affection near to us by lively imagery and representation, and make it look like truth and reality. (V.II.15)
Substitute "poetry" for music, movies, or TV shows, and this statement suddenly becomes surprisingly relevant to our day-to-day lives.