Sheep are one of the only animals that can’t really take care of themselves. All of the dirt, grime, and dust naturally gets absorbed into their wool, and they need someone to clean them. Left to themselves, sheep would wander off into the wilderness and die. They need a someone to guide them. A sheep can't really protect itself, and it can't really provide for itself. Sheep need a shepherd.
Isaiah 53:6a says, we all like sheep have gone astray; we have turned—every one—to his own way. This is the human condition. We, like sheep, wander off on our own path leaning on our own understanding - trusting in something other than the Good Shepherd to save, sustain, and satisfy.
Psalm 23 is King David's deeply personal testimony of God's goodness and faithfulness throughout his life. As a Shepherd King, David knew what it meant to say of his creator “The LORD is my Shepherd”