NOTES for Num 21:1-35
The way to the promised land is full of adventures, and many of them, like the story with the snakes that attacked the people after another outburst of indignation, are troubles the travelers bring on themselves. And no wonder. One can follow God into the land He promised only by following Him literally step by step. That, for the most part, is how it was - it is no accident that there, in the Book of Numbers, it is mentioned that everyone watched the Tabernacle and the cloud of God's presence above it. When the cloud moved, the people moved too. Without God's instruction no one went anywhere - at least outwardly. Inwardly it was more complicated. The inner state is not so easy to recognize; here everything is not so obvious or unambiguous.
One can follow God - but what will be in the heart while doing so? If a person on the way after God becomes extremely demanding, it means that inwardly he experiences his movement as a kind of favor to God, who, since He has called him to follow, must also guarantee some acceptable minimum of comfort, and the person usually considers himself entitled to define the measure of what is acceptable. If right now we want to drink, then there must be water somewhere nearby, and God must take care of it. If He does not take care of it, then He has either abandoned us or deceived us from the beginning.
For a fallen human being such a reaction of distrust is quite understandable and natural - it is simply the transfer of relationships between people in the fallen world onto God and His relationship to us. Simply put, from Him we expect the very same thing that, consciously or unconsciously, we have become accustomed to expect from the people around us. An a priori distrust of fallen humanity is understandable, but that does not make it easier: it interferes very strongly with one's relationship with God, in fact destroying it as soon as it begins to be established.
The only way to resist this, when it concerns fallen humanity, is to place a person in conditions of real danger, where he finds himself face to face with someone or something that truly threatens him. Then a person usually forgets all his prejudices and rushes to God. So it was with the people during skirmishes with the nations and tribes through whose territory they had to pass: there one had either to trust God for real or perish. In relatively peaceful times, the role of enemies was played by those very snakes: they forced a person to remember God as the only One who can help and save. The pedagogy is severe, but the situation is critical: the question of trusting God in the desert is a question of survival, not only spiritual, but also physical.