Ecclesiastes:

This, in general, is a pessimistic book -- with statements that history merely repeats itself and nothing is new, and there is no reason to think of what might have been.

The most famous section is Ecclesiastes 3:1-8 (from King James' Version):

1. To everything there is a season, and a time to every purpose under the heaven:
2. A time to be born, and a time to die; a time to plant, and a time to pluck up that which is planted;
3. A time to kill, and a time to heal; a time to break down, and a time to build up;
4. A time to weep, and a time to laugh; a time to mourn, and a time to dance;
5. A time to cast away stones, and a time to gather stones together; a time to embrace, and a time to refrain from embracing.
6. A time to get, and a time to lose; a time to keep, and a time to cast away;
7. A time to rend, and a time to sew; a time to keep silence, and a time to speak;
8. A time to love, and a time to hate; a time of war, and a time of peace.

Ecclesiastes states that people should enjoy themselves and the fruits of their labors. The pessimism is most pronounced when considering man and animals returning to dust, and not knowing where their spirits go (Ecclesiastes 3:18-21). A few other points:

1. "Whoever loves money never has money enough; whoever loves wealth is never satisfied with his income. This too is meaningless." (Ecclesiastes 5:10).
2. "... to accept his lot and be happy in his work -- this is a gift of God..." (Ecclesiastes 5:19-20)
3. "Better what the eye sees than the roving of the appetite (i.e., dreaming). This too is meaningless, a chasing after the wind." (Ecclesiastes 6:9).
4. On predestination: "Whatever exists has already been named, and what man is has been known; no man can contend with One who is stronger than he (i.e., can fight with God)." (Ecclesiastes 6:10)
5. "...the day of death better than the day of birth." (Ecclesiastes 7:1)
6. "...all that God has done. No one can comprehend what goes on under the sun...man cannot discover its meaning. Even if a wise man claims he knows, he cannot really comprehend it." (Ecclesiastes 8:17)
7. "Wisdom is better than weapons of war, but one sinner destroys much good." (Ecclesiastes 9:18).
8. "Whoever watches the wind will not plant; whoever looks at the clouds will not reap (i.e, if you want perfection, you won't do anything.)." (Ecclesiastes 11:4).
9. "As you do not know the path of the wind, or how the body is formed in a mother's womb, so you cannot understand the work of God, the Maker of all things." (Ecclesiastes 11:5)
10. "...the conclusion of the matter: Fear God and keep His commandments, for this is the whole duty of man. For God will bring every deed into judgment, including every hidden thing, whether it is good or evil." (Ecclesiastes 12:13-14).