Albert Barnes Commentary Psalms 78:18

"And they tempted God in their heart By asking food according to their desire." — Psalms 78:18 (ASV)

And they tempted God in their heart - (Exodus 16:2). The heart was the source of the evil. They were not satisfied with what he gave them. They asked for what would be more agreeable to them, and they did so with a complaining and murmuring spirit. It is not wrong in itself to ask God for what will be better than what we now possess, for that is the object of all our prayers; but this may be done from a wrong motive—for mere self-gratification, as was the case here; or it may be with a complaining and dissatisfied spirit, such as was shown on this occasion. In such a case, we cannot expect the prayer to be answered "except as a punishment."

By asking meat for their lust - Food. The word "meat" here does not necessarily denote animal food, as it commonly means to us. They asked for a different kind of food than manna; and they did so, not because this was "necessary" to sustain life, but in order to gratify their appetites. The original word here, however, is not "lusts," but "souls;" that is, "they asked food for themselves."