Sheep obey, follow, trust, love, and depend upon their shepherd. So too, we have been bought by our Shepherd. If divine promises are to help us, we must embrace them. David, of course, was a shepherd, and so knew from experience the care a shepherd must give to his flock. The shepherd rules, guides, and protects his flock.
Good shepherds show great care for their sheep.
David also knew of the humbling, degrading tasks that a shepherd must do, as he cares for his helpless, stupid sheep. It is because of our weaknesses that the Lord has to condescend to be a shepherd, rather than some more noble profession. This statement follows from the first. There is absolutely nothing good that I shall lack, for body, soul or spirit. Not that I will have all of my foolish desires fulfilled, but that I shall not be in want for anything necessary and good.