"Now when the people complained, it displeased the LORD; for the LORD heard it, and His anger was aroused. . . . "
There is an acceptable (even proper) way to "complain" to God (cf. Ps. 142:2); and then there is this unacceptable and improper way to do it, (as seen all throughout Num., ch. 11). The people "craved"; they wanted their way; they "wept all over" Moses; they insisted that they be granted their desires. God's answer was to give them exactly what they wanted, and then to afflict many of them, (see vv. 33-34).
Let us be careful about what we earnestly long for; and of what we "demand" of God. In fact, it may not be good for us. The "status quo," (viz. continuing to eat God's provision of His manna), is a whole lot better than an early grave.
[Puritan quote of the day: "Hence we learn that a life of holiness is the pleasantest life in this world, because in such a life we have the imperfect beginnings of a blessed and endless sight of God." --Jona-
than Edwards, in his sermon, "The Pure in Heart Blessed"]