God Knows Our Weakness
Psalm 78:39 (ESV) … “He remembered that they were but flesh, a wind that passes and comes not again.”
What a speech the psalmist makes of this period. In reading Psalm 78 the writer divides the problem into three parts. He tells of the people tempting God in three ways: in connection with the lust of the flesh, the lure of the world, and the lies of the devil.
They tempted God in connection with the lust of the flesh: “And they sinned yet more against Him by provoking the Most High … in their heart, by asking meat for their lust. Yea, they spake against God; they said, Can GOD furnish a table in the wilderness? Can He provide flesh for his people?” (78:17–20).
He had given them the manna—what the psalmist calls “angel’s food” (paraphrased in the Septuaguint as “the bread of the mighty”). He had riven the rock and given them water, and they insolently asked if He could give them flesh to eat. Of course He could! The southeast wind brought up quail from the sea. It was springtime and the birds were migrating northward from Africa, following their usual path up the coast of the Red Sea to the Sinai Peninsula, where they naturally crossed at the narrow end, just where Israel was encamped. The people fell upon the quail which rained down upon them but, before they could surfeit their lust, the judgment of God fell upon them.: “While their meat was yet in their mouths the wrath of God came upon them.”
They had provoked God Most High, El Elyon. It was thus that Abraham had acknowledged Him when seated at the table with Melchizedek. Yet God did not destroy Israel utterly. He remembered their frailty: “Yea, many a time turned He His anger away and did not stir up all His wrath. For He remembered that they were but flesh, a wind that passeth away, and cometh not again” (78:38–39). 
That they were human; that they were weak; that they were prone to err; that they were liable to fall into temptation. In his dealings with them he took into view their fallen nature; their training; their temptations; their trials; their weaknesses; and he judged them accordingly. Comp. Ps. 103:14. So it was with the Savior in his treatment of his disciples, “The spirit indeed is willing, but the flesh is weak,” Matt. 26:41. God will judge men as they are; he will not in his judgments forget that they are men, and that they are weak and feeble. Men often judge their fellow-men with much more harshness, with much less allowance for their infirmities and weaknesses, than God shows in his dealings with mankind. And yet such are the very men who are most ready to blame God for his judgments. If God acted on the principle and in the manner according to which they act, they could hope for no mercy at his hand. It is well for them that there is not one like themselves on the throne of the universe.
A wind that passeth away, and cometh not again. Which blows by us, and is gone for ever. What a striking description is this of man! How true of an individual! How true of a generation! How true of the race at large! God remembers this when he thinks of men, and he deals with them accordingly. He is not harsh and severe, but kind and compassionate. To man, a being so feeble,—to the human race, so frail,—to the generations of that race, so transitory, so soon passing off the stage of life,—he is ever willing to show compassion. He does not make use of his great power to crush them; he prefers to manifest his mercy in saving them. 
Phillips, J. (2009). Exploring Psalms 1–88: An Expository Commentary (Vol. 1, Ps 78:17–66). Kregel Publications; WORDsearch Corp. Barnes, A. (1870–1872). Notes on the Old Testament: Psalms (Vol. 2, p. 301). London: Blackie & Son.