Nothing New!
1:9  The thing that hath been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun.

All that is done under the sun is vanity and vexation of spirit!
1:14  I have seen all the works that are done under the sun; and, behold, all is vanity and vexation of spirit.

All that HE did was also vanity and vexation of spirit!
2:11  Then I looked on all the works that my hands had wrought, and on the labour that I had laboured to do: and, behold, all was vanity and vexation of spirit, and there was no profit under the sun.

Therefore, he hated life!
2:17-22  Therefore I hated life; because the work that is wrought under the sun is grievous unto me: for all is vanity and vexation of spirit.  Yea, I hated all my labour which I had taken under the sun: because I should leave it unto the man that shall be after me.   And who knoweth whether he shall be a wise man or a fool?  yet shall he have rule over all my labour wherein I have laboured, and wherein I have shewed myself wise under the sun.  This is also vanity.
Therefore I went about to cause my heart to despair of all the labour which I took under the sun.   For there is a man whose labour is in wisdom, and in knowledge, and in equity; yet to a man that hath not laboured therein shall he leave it for his portion.  This also is vanity and a great evil.   For what hath man of all his labour, and of the vexation of his heart, wherein he hath laboured under the sun?

He saw wicked judgment and iniquity was in the righteousness!
3:16  And moreover I saw under the sun the place of judgment, that wickedness was there; and the place of righteousness, that iniquity was there. See also Proverbs 29:2,12; 28:15 - then see "Jesus" deal with this in Isaiah 14:5

He saw oppressions and those that were oppressed - they had no comforter against the power of the oppressers!  (these are our fair goers, etc.)
4:1   So I returned, and considered all the oppressions that are done under the sun: and behold the tears of such as were oppressed, and they had no comforter; and on the side of their oppressors there was power; but they had no comforter.  Wherefore I praised the dead which are already dead more than the living which are yet alive.  Yea, better is he than both they, which hath not yet been, who hath not seen the evil work that is done under the sun.  Also see Romans 16:19 for our 'level' of awareness towards evil today...

He recognized a specific evil that is common among men - riches to have 'no want' for his soul, etc. from God but to not be given the power to eat thereof.
6:1  There is an evil which I have seen under the sun, and it is common among men:  A man to whom God hath given riches, wealth, and honour, so that he wanteth nothing for his soul of all that he desireth, yet God giveth him not power to eat thereof, but a stranger eateth it: this is vanity, and it is an evil disease.

No eternal future let alone what is good for them now in this life under the sun!
6:12  For who knoweth what is good for man in this life, all the days of his vain life which he spendeth as a shadow?  for who can tell a man what shall be after him under the sun?

We can!!!

All must eventually work for someone!  In other words, SUBMIT!
8:9  All this have I seen, and applied my heart unto every work that is done under the sun: there is a time wherein one man ruleth over another to his own hurt.

Nothing better to do other than eat and drink and be merry by (earned money) the work done under the sun!
8:15  Then I commended mirth, because a man hath no better thing under the sun, than to eat, and to drink, and to be merry: for that shall abide with him of his labour the days of his life, which God giveth him under the sun.

Couldn't sleep - and couldn't find the work of God.
8:16,17  When I applied mine heart to know wisdom, and to see the business that is done upon the earth: (for also there is that neither day nor night seeth sleep with his eyes:)  Then I beheld all the work of God, that a man cannot find out the work that is done under the sun: because though a man labour to seek it out, yet he shall not find it; yea further; though a wise man think to know it, yet shall he not be able to find it.
I Cor 2:9-16 - only by the Holy Spirit can we see the deep things of God.

It all eventually passes away!  Any gain is gone!
9:5,6  For the living know that they shall die: but the dead know not any thing, neither have they any more a reward; for the memory of them is forgotten.  Also their love, and their hatred, and their envy, is now perished; neither have they any more a portion for ever in any thing that is done under the sun.
Except for what work was done for God... and at that, there is still judgment coming - judgment for sin or good/bad works... saved or lost...

He sees it as all time and chance - no purpose in life.
9:11  I returned, and saw under the sun, that the race is not to the swift, nor the battle to the strong, neither yet bread to the wise, nor yet riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all.

Romans 12:2 - the Lord DOES a purpose - a WILL for each of us!

It must be someone else's fault - the leaders, etc. - maybe God's?
10:5  There is an evil which I have seen under the sun, as an error which proceedeth from the ruler: