Albert Barnes Commentary Psalms 103:15

"As for man, his days are as grass; As a flower of the field, so he flourisheth." — Psalms 103:15 (ASV)

As for man - literally, “Man; like the grass are his days!” The thought is fixed on man: man so frail and weak; man, not only made originally of earth, but man delicate, feeble, soon to pass away like the springing grass, or like the fading flower.

His days are as grass - See the notes at (Psalms 90:5–6); compare (Isaiah 40:6–8), notes; (1 Peter 1:24), note.

As a flower of the field - As a blossom. It opens with beauty and fragrance, but soon fades and perishes.

So he flourisheth - Rather, “So he blossoms.” That is, he is like a flower that is fresh and beautiful, and that soon withers away.