Beha'aloscha: Man's Search for Meaning
The Torah relates that the people (ha’am) were as if in mourning (Numbers 11:1), but does not say what they were displeased about.
The commentaries say that these people (ha’am) were the Egyptians that joined the Israelites in the exodus. They were trouble makers all along. They were the ones that made the Golden Calf, challenged G-d to produce water in the desert, and advocated return to Egypt. Now they were just griping for no apparent reason. They were unhappy with the Torah, and had no ultimate goal in life.
If a person does not know what one is living for, one will be chronically discontented, and will look for things to blame for the discontent. Such people are likely to turn to any of the addictions, which can give them momentary “highs,” which will only add to their long-term misery.