In the first verse of the twenty-third psalm, David says: The LORD is my shepherd, I shall not want. To my mind, the second clause is an affirmation grounded in the first. Because the LORD is my shepherd, consequently I will lack for nothing. But now we have to ask the question: is this true? Haven't we all experienced times of lack, times of great need, when God did not exactly seem quick to provide? And yet David is confident that he will not be in need of anything, since God is shepherding him.
The other night during my Foundations for a Spiritual Life course, the professor asked us the question: what are the characteristics of a healthy intimate relationship? Among other things, I named trust -- trust that the other person will not do us harm or react poorly if we should say something unpleasant, etc. The professor added to this that there is offered the benefit of the doubt: if the other person said something or did something questionable, we give them the benefit of the doubt; we assume the best. For instance, if something in scripture seems questionable or outrageous even, we offer God the benefit of the doubt.
It may very well be that for us to have an intimate relationship with God, for us to have a healthy sheep-shepherd relationship with the LORD, we have to give him the benefit of the doubt. When David says I shall not want, that is not a scientific statement of straightforward and event empirical fact. It is a statement of faith, it is an affirmation of his trust in God. The LORD will take care of him and provide for him all that he needs in the right time. David trusts God.
We have a natural human tendency to assume that we know better, that we see enough of the relevant facts in any scenario to make a grounded and educated judgment about any matter. When we lack something we want (maybe even an essential of life) and we can see no compelling reason why we shouldn't have it, we draw the inference that consequently there couldn't be any reason why we shouldn't have it. We get angry at God, we may deny that he loves us or that he cares for us, and we may even deny his existence in the end.
If anything, I think one of the lessons of the scriptures is that we are not capable of making these kinds of judgments. God's vision is grand and all-encompassing as omniscient creator, whereas we are specks of dust swirling about on planet Earth. Why not rather trust him? Why not rather trust that there is a reason for the things that are happening to us? Why suppose that we should be privy to the reason if there were one? How many times has our assumption of better knowledge been proven wrong in the case of the actions of other humans? Let alone with God!