I don't just mean they memorized it. They were wired into it. I have been with stroke patients who could hardly put a sentence together without getting confused, but when I started reciting the twenty third psalm, they joined right in and didn't miss a word. It was something of a shocking image in those days because shepherds were the lowest rung of the social ladder, a rough and smelly lot. And when the Bible compares us to sheep, it's not really a compliment.
Sheep are notoriously stupid, self willed, driven by their appetites.
We don't think long of shepherd and sheep. We imagine green pastures, still waters, good directions, God making us lie down, God leading us, God caring for us with tender mercies. What a sentence. The valley, because we feel low and diminished. The shadow, because the fear of death kills us before the reality of death arrives.