I can't find the precise quotation at the moment, and when I do I'll post it, but the great poet and Shakespeare commentator Samuel Coleridge once wrote something about the goal of poetry being to turn the reader into an active creative agent. The thought not only goes a long way toward explaining why poetry is often considered difficult, but also why, paradoxically, the longest-lasting of the arts is usually the least regarded (or popular) in its own time: it requires a great deal from its audience. Of course the enduring and ever-increasing nature of poetry is one of Shakespeare's great themes, mentioned most frequently in the sonnets, but put into practice most fully in the plays, where it is virtually impossible to read more than a page or two without encountering an astonishing poetic image. One example that I came across recently is found in the opening lines of the first scene from act four of Henry VI, Part Two, wherein the captain of a ship carrying prisoners, including the disguised Earl of Suffolk (who is soon to discovered and beheaded), gives this grisly description of nightfall, which he personifies as being drawn in a chariot by "jades", which in this case seems to refer not to overworked horses (as is usually the case), but to low-flying dragons:

The gaudy, blabbing and remorseful day

Is crept into the bosom of the sea;

And now loud-howling wolves arouse the jades

That drag the tragic melancholy night;

Who, with their drowsy, slow and flagging wings,

Clip dead men's graves and from their misty jaws

Breathe foul contagious darkness in the air.