Kibroth Hattaavah means "the graves of greediness." Their sin was not just in giving in to their craving. Their sin was they doubted God's ability to supply and they doubted His concern for their welfare.
Understand that God's concern for us is just as great after His calling as it is before. He is still working out His purpose, and He will supply our need. Remember, though, when God gives us what we desire and pray for, it does not necessarily mean that it is a blessing, as in this situation when the "blessing" turned out to be the instrument of death. It is a sobering lesson to keep in the forefront of our minds. Our prayer should always be, "Not my will but Yours be done. God, please remember I am just human."
Human nature is never satisfied. It is filled with self-concern and does not know what is best for it. What it lusts for may even lead to that person's spiritual death. It makes us think that the grass is greener on the other side and that there is more and better in something else, something new and exciting. And when lust is involved, anticipation is always greater than realization. There is a law of diminishing returns at work in this universe that perversion lessens rewards. The Israelites had a perverse craving for tasty food, and their reward ended up being death. Human nature is something we are always going to have to deal with in this life.
God was not dealing with these people in terms of salvation as He is with us. The lesson for us is not to let these cravings—even desires for good things—take our eyes off the goal and the reality of what God is doing for us.
Jeremiah 10:23-24 says that the way of man is not in him to direct his steps. We have to understand that, when we come to God, we are admitting to Him through repentance that our salvation is not internal—it is not something we can produce. In the same vein, the right way to live is not within us. It must come from outside, and that "outside" is God. Thus, we ask God to direct our steps. At baptism, we are asking God to make us into the image of Christ and to rid us of the perversions of human nature that have produced this world.
The experience of the Israelites shows us that, when the going gets unexpectedly rough and hardships occur—say, in the area of tithing, that we have not been blessed to the extent we feel we deserve, or in the area of Sabbath, that we lose our job and cannot find another—and then we have an intense craving for something and begin to look back at our former situation, we can also begin to lust for the very things that not long before we considered to be expendable and holding us in bondage.