Sheep have a special problem. They have no defenses. Cats have teeth, claws and speed. Dogs have their teeth and their speed. Horses can kick, bite and run. Bears can claw, bite and crush. Deer can run. But the sheep have no bite or claws and cannot outrun any serious predator. They can butt other sheep, but that ability will not protect them from a wolf or a bear. The sheep’s only security is the shepherd.
Kenneth Bailey, The Good Shepherd: A Thousand Year Journey from Psalm 23 to the New Testament, chapter one
Locus Communis 