John Gill 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, his days are as grass
He himself is like the grass which springs out of the earth; continues on it for a time, and then drops into it; the continuance of the grass is very short, it flourishes in the morning, is cut down at evening, and withers; see (Psalms 90:5Psalms 90:6) .

As a flower of the field, so he flourishes; which denotes the goodliness of man, and describes him in his best estate, as possessed of health, riches, honour, and all the gifts and endowments of nature; and yet, with all these, is only like a field flower, exposed to every wind, liable to be cropped by every hand, and to be trampled upon by the beasts of the field; and therefore flourishes not long: so very precarious and uncertain is man in his most flourishing circumstances; see (Isaiah 40:6–8) .