Jamieson, Fausset and Brown on Ecclesiastes 8:16

16. Reply to Ec 8:14, 15. When I applied myself to observe man's toils after happiness (some of them so incessant as not to allow sufficient time for "sleep"), then (Ec 8:17, the apodosis) I saw that man cannot find out (the reason of) God's inscrutable dealings with the "just" and with the "wicked" here (Ec 8:14; Ec 3:11; Job 5:9; Ro 11:33); his duty is to acquiesce in them as good, because they are God's, though he sees not all the reasons for them (Psa 73:16). It is enough to know "the righteous are in God's hand" (Ec 9:1). "Over wise" (Ec 7:16); that is, Speculations above what is written are vain.