He guides us by the Holy Spirit and He is constantly caring for us. This is life support for the sheep. Many things about sheep correlate with humans magnificently. The strange thing about sheep is that it's almost impossible for them to lie down and rest unless certain conditions are present, there must be a definite sense of security and safety and freedom from fear, tension, aggravations and hunger. Therefore, God provides everything that His sheep need to give them rest.
Sheep require a consistent amount of water.
However, unlike other animals they are totally dependent on their shepherd to find this water. God provides the quiet waters which we need to survive. Having the soul restored means that even though we are in the care of the good shepherd, we have times when we need restoration. Here the Hebrew word means to bring back home again, recover, refresh, relieve, retrieve, return. David said this because our soul needs to be restored, including our mental attitude.