It is impossible for anyone with perfect righteousness to lack anything. The justice of God sends forth life support and blessing to both winner and loser believers. It just so happens that the one who wrote this Psalm under the ministry of the Holy Spirit was a winner. For the Lord is the one shepherding me. Remember sheep do not just take care of themselves as other animals do.
Sheep require endless attention and personal care.
The Lord Jesus Christ as our good Shepherd laid down His life for us. 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.