John Gill Commentary Psalms 78:39

"And he remembered that they were but flesh, A wind that passeth away, and cometh not again." — Psalms 78:39 (ASV)

For he remembered that they [were but] flesh
Or "children of flesh", as the Targum; poor, frail, weak, mortal creatures, unable to bear the weight of his displeasure, the stroke of his hand, and the lighting down of his arm, with the indignation of his wrath; that they must be crushed before him, and would sink, and fail, and die; see (Psalms 103:14) (Isaiah 57:16) ,

or that they were naturally sinful and corrupt, prone to evil, easily drawn into sin; it was what their depraved natures inclined unto; they were impotent to that which is good, and unable to withstand temptations to evil; all which was taken notice of and considered by the Lord in his condescending goodness, and therefore he dealt gently with them; see (Genesis 6:3) (8:21) ,

a wind that passeth away, and cometh not again ;
such is the life of man; it may be fitly compared to the wind, which moves swiftly, and, passing on, loses its strength and subsides; so the life of man is quickly gone, his days move swiftly on, he dies, and returns not again to his former state, to a mortal life; and though the spirit returns to the body again, yet not till the resurrection; and then not of itself, but by the power of God; see (Job 7:7) .