What is the source of its appeal?
No doubt it is due to the imagery of the loving shepherd caring for the sheep. But there is also a subtle shift in the poem that enhances its emotional intimacy and potency. But it's a subtle shift. Can you see it?
The Lord is my shepherd;The poet begins by addressing a human audience and referring to God in the third person:
I shall not want.
He makes me to lie down in green pastures;
He leads me beside the still waters.
He restores my soul;
He leads me in the paths of righteousness
For His name’s sake.
Yea, though I walk through the valley of the shadow of death,
I will fear no evil;
For You are with me;
Your rod and Your staff, they comfort me.
You prepare a table before me in the presence of my enemies;
You anoint my head with oil;
My cup runs over.
Surely goodness and mercy shall follow me
All the days of my life;
And I will dwell in the house of the Lord
The Lord...The imagery is powerful and rich but it's held at a distance. Describing another time and place and a person who is not present.
He makes me...
He leads me...
He restores my soul.
And then, suddenly but subtly, so subtly that you don't really notice it, the focus shifts away from a human audience to address God directly. "He" shifts to "you."
You are with me...It's a shift--the change from "he" to "you"--that we hardly consciously register. But the emotional effect is one of deepening intimacy. The poem starts with God held at a distance and then, almost imperceptibility, it becomes a very personal, intimate, direct and face-to-face engagement.