The metaphor of the king as the shepherd of his people goes back to ancient Egypt. Perhaps the use of this particular convention is due to the fact that, being stupid, affectionate, gregarious, and easily stampeded, the societies formed by sheep are most like human ones.
Northrop FryePoetry is the most direct and simple means of expressing oneself in words: the most primitive nations have poetry, but only quitewell developed civilizations can produce good prose. So don't think of poetry as a perverse and unnatural way of distorting ordinary prose statements: prose is a much less natural way of speaking than poetry is. If you listen to small children, and to the amount of chanting and singsong in their speech, you'll see what I mean.
Northrop Frye