C. F. Keil and Franz Delitzsch on Ecclesiastes 3:9

Ecc 3:9

Since, then, everything has its time depending not on human influence, but on the determination and providence of God, the question arises: “What gain hath he that worketh in that wherewith he wearieth himself?” It is the complaint of Ecc 1:3 which is here repeated. From all the labour there comes forth nothing which carries in it the security of its continuance; but in all he does man is conditioned by the change of times and circumstances and relations over which he has no control. And the converse of this his weakness is short-sightedness.