In "The Lamb," nature is a product of God, the ultimate shepherd. When God says, "Eat that grass," you eat that grass. When God says, "Wander by that stream," you do that too. Oversimplification? Perhaps. But the point is that the natural world as depicted in this poem isn't a particularly savage or unpredictable place. What's more, nature is the means by which we learn about God. The poem implies that all the nice things in the world prove that God is a kind benefactor who will guide us innocent lambs through the sunny world.
"The Lamb" shows that Blake believes in a "clockwork God," who – having set the universe in motion – has withdrawn from his creation.