From Mount Hor they set out by the way to the Red Sea, to go around the land of Edom; but the people became impatient on the way. The people spoke against God and against Moses, "Why have you brought us up out of Egypt to die in the wilderness? For there is no food and no water, and we detest this miserable food." Then the Lord sent poisonous serpents among the people, and they bit the people, so that many Israelites died.  

The people came to Moses and said, "We have sinned by speaking against the Lord and against you; pray to the Lord to take away the serpents from us." So Moses prayed for the people. And the Lord said to Moses, "Make a poisonous serpent, and set it on a pole; and everyone who is bitten shall look at it and live." So Moses made a serpent of bronze, and put it upon a pole; and whenever a serpent bit someone, that person would look at the serpent of bronze and live.

They had been freed.  Not only had they been freed from slavery in Egypt, they had been freed from worrying about where their next meal was going to come from.  Who was going to fill the gas tank, get to the store, pay for the groceries, cook the manna and serve it?  No worries!  God had taken care of all of that. Still, they griped.

After the serpents were sent in, the God who had freed them from slavery freed them from death, too.  This had the salutary effect, at least momentarily, of focusing all of the complaining on one truly serious common problem - death by snake bite.  With that behind them, one would think that God's people might have perceived their healing as an apt occasion for a major complaint-upgrade, exchanging their random nattering for some good, strong, unified lamenting over the sorts of things that caused God real grief.  One would think. . .