KEY THOT: In this well-known psalm, David sees the Lord as his Shepherd. Being a shepherd himself, David understands the shepherd's heart and responsibility to lead the flock to "green pastures", "still waters" and "paths of righteousness". And because the Lord was his shepherd, David boldly declares that he "shall not want"--that is, lacking in nothing that is good for him. Implied in this shepherd imagery is the assumption of the sheep's instinct to listen and obey the shepherd's voice.
Every sheep recognizes his own shepherd's voice and will respond immediately to his call. He won't respond to a stranger's voice. This assumption of obedience ensures that sheep will enjoy the shepherd's protection (rod) and rescue (staff). The sheep needs to stay close to the shepherd to be able to hear his voice when he calls. Only then will the sheep lacks nothing good and enjoy the Shepherd's provision and protection all the days of his life: "goodness and mercy shall follow me all the days of my life" (v.6).
The converse is equally true: if a sheep chooses to ignore the shepherd's voice and wanders off on his own, then he exposes himself to dangers (wolves) and also to hunger and thirst. While away from the shepherd, the sheep will not be able to enjoy the shepherd's rod of protection and the staff of rescue when he falls into the ravines. God's grace never means the sheep can ignore the shepherd's voice and still enjoy God's goodness and mercy. God's grace implies that God's free blessings are available if we stay close to Him. Psalms 91:1 says, "Those who live in the shelter of the Most High will find rest in the shadow of the Almighty" (NLT).
If grace implies we will not suffer any negative consequences of our own disobedience, how then do we explain why Christians get into all kinds of troubles -- moral, spiritual and relational? Obviously, our enjoyment of God's goodness and mercy depends very much on our ability and willingness to follow God's voice, rather than our own desires or Satan's enticements.
Jesus himself has said, "My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand" (John 10:27-28). Many Christians like to use John 10:28 to reassure themselves of their own eternal security in Christ by declaring that "no one will snatch them out of my hand." But they have conveniently ignored the condition for this promise given in preceding verse: "My sheep hear my voice... and they follow me."
David emphasizes God's leadership in his life in this psalm: "He leads me beside still waters... He leads me in paths of righteousness for his name's sake" (verses 2-3). Leaders are to be followed and obeyed. And the Lord is our Leader and we cannot enjoy his blessings of "green pastures", "still waters" and "paths of righteousness" if we choose to wander off on our own. To expect otherwise is pure foolishness.