Sheep are animals that are much different from other animals. Every other kind of animal can exist in the wild. A sheep has no sense of direction, and no way to protect itself.
Man is like a sheep.
He was created with a need to depend upon God for his preservation and direction. Everything God created during the seven creative days He created for mankind. But man was created for God. David was a shepherd since his early youth, and understood about sheep and shepherds. He knew that his sheep depended upon him for all of their needs and preservation.