David not only knew what it was like to face danger as a shepherd, but he had also experienced many painful times in his personal life. And yet, he knew that fear should not control someone who knows the Lord and has experienced His mercy and help. So David writes about "walking through the valley of the shadow of death", but he chooses to not live in fear, because he knows the strength of the protection and promises of God. It is a powerful reassurance for anyone struggling through a difficult situation that our Shepherd is our Defense and provides guidance to us.