There are only two negatives in the Twenty-Third Psalm. "I shall not want" and "I will fear no evil". Yet, we're prone to doing just those things the Psalmist says he won't do -- living in fear, and wanting what we don't have. If we are the Lord's sheep we have all we need to live confident contented lives.
What exactly does it mean to "not want"? Certainly part of the meaning is that we'll lack for nothing, but it's more than that. It's easy to miss the plain meaning of the word "want". Here's more from Douglas MacMillan's exposition of Psalm 23 . . .
. . . this word means more than mere lack: it means just what the word 'want' originally meant. It means that I will not be discontented with my lot; I will not be hungering and craving after things that God has forbidden me, because I will find my all, my fulness, in the One who is my Shepherd. My knowledge that I shall suffer no lack will give me contentment . . . . One of the things which is fundamental to the whole business and profession of a shepherd is this: enough passion to see that his sheep will have all that they need, and enough sense to see that they will not get what will harm or destroy them. That is the kind of shepherd, and that is the kind of satisfaction, that the Christian believer finds in Christ. 'I shall not want.'
MacMillan suggests that when we find contentment and satisfaction in Christ lacking -- when we find ourselves wanting -- it's because our eyes have been drawn away from the promises of God's Word. He encourages the reader to "use your Scripture, allow God to fortress and garrison your heart with the great strength of His promises and of His logical grace."
Another way to stay close to our shepherd is to stay close to his flock. Christian fellowship and regular attention to the ordinary means of grace (the reading and preaching of the word, prayer, and the sacraments) are the things that keep us anchored to our hope in Christ (Heb. 6:19).
Quotes from J. Douglas MacMillan, The Lord Our Shepherd (pp. 50-1)