If a man live many years - And even have prosperity through the whole; yet the days of darkness - times of affliction, weakness, and perhaps old age, will be many. If he die not a violent death, which no man can wish, he will die a lingering death; and this is ordinarily attended with many pains, and many sorrows; therefore let him prepare to meet his God; and to carry this thought through life, that all must terminate in death. The writer of Ecclesiasticus, 7:36, has a good saying, similar to this: "Whatsoever thou takest in hand, remember thy End; and thou shalt never do amiss;" ουκ ἁμαρτησεις, thou wilt not sin.

if a man

Ecclesiastes 6:6 Yes, though he live a thousand years twice told, yet has he seen no good: do not all go to one place?

Ecclesiastes 8:12 Though a sinner do evil an hundred times, and his days be prolonged, yet surely I know that it shall be well with them that fear God...

rejoice

Ecclesiastes 3:12,13 I know that there is no good in them, but for a man to rejoice, and to do good in his life...

Ecclesiastes 5:18-20 Behold that which I have seen: it is good and comely for one to eat and to drink...

Ecclesiastes 8:15 Then I commended mirth, because a man has no better thing under the sun, than to eat, and to drink, and to be merry...

yet

Ecclesiastes 7:14 In the day of prosperity be joyful, but in the day of adversity consider: God also has set the one over against the other...

Ecclesiastes 12:1-5 Remember now your Creator in the days of your youth, while the evil days come not, nor the years draw near, when you shall say...

Deuteronomy 32:29 O that they were wise, that they understood this, that they would consider their latter end!

Job 10:22 A land of darkness, as darkness itself; and of the shadow of death, without any order, and where the light is as darkness.

Job 14:10 But man dies, and wastes away: yes, man gives up the ghost, and where is he?

Job 15:23 He wanders abroad for bread, saying, Where is it? he knows that the day of darkness is ready at his hand.

Job 18:18 He shall be driven from light into darkness, and chased out of the world.

Jeremiah 13:16 Give glory to the LORD your God, before he cause darkness, and before your feet stumble on the dark mountains, and...

Joel 2:2 A day of darkness and of gloominess, a day of clouds and of thick darkness, as the morning spread on the mountains...

Matthew 22:13 Then said the king to the servants, Bind him hand and foot, and take him away, and cast him into outer darkness...

John 12:35 Then Jesus said to them, Yet a little while is the light with you. Walk while you have the light, lest darkness come on you...

Jude 1:18 How that they told you there should be mockers in the last time, who should walk after their own ungodly lusts.

all that

Ecclesiastes 2:1-11,15,17,19,21-23,26 I said in my heart, Go to now, I will prove you with mirth, therefore enjoy pleasure: and, behold, this also is vanity...

Ecclesiastes 4:8,16 There is one alone, and there is not a second; yes, he has neither child nor brother: yet is there no end of all his labor...

Ecclesiastes 5:15,16 As he came forth of his mother's womb, naked shall he return to go as he came, and shall take nothing of his labor...

Ecclesiastes 6:11 Seeing there be many things that increase vanity, what is man the better?

Curiosity a Temptation to Sin.
"Enter not into the path of the wicked, and go not in the way of evil men. Avoid it, pass not by it, turn from it, and pass away."--Proverbs iv. 14, 15. The chief cause of the wickedness which is every where seen in the world, and in which, alas! each of us has more or less his share, is our curiosity to have some fellowship with darkness, some experience of sin, to know what the pleasures of sin are like. I believe it is even thought unmanly by many persons (though they may not like to say …
John Henry Newman—Parochial and Plain Sermons, Vol. VIII

Ecclesiastes
It is not surprising that the book of Ecclesiastes had a struggle to maintain its place in the canon, and it was probably only its reputed Solomonic authorship and the last two verses of the book that permanently secured its position at the synod of Jamnia in 90 A.D. The Jewish scholars of the first century A.D. were struck by the manner in which it contradicted itself: e.g., "I praised the dead more than the living," iv. 2, "A living dog is better than a dead lion," ix. 4; but they were still more …
John Edgar McFadyen—Introduction to the Old Testament