Ecclesiastes

Vanity of vanities, all is vanity.

While many (including the preacher) interpret this as an explanation of the endless cycle of meaningless life on earth and the fact that because nothing you can possibly think or do is original and it all ends in the same inevitable death, it is all pointless. Everyone dies, and everything you do between now and then is in vain. For some this is justification for pursuing a life of sinful debauchery, indulging themselves for lack of anything meaningful to do, but as the third chapter explains, there is a time for every purpose under heaven. To me, this is God's reassurance that life on earth is not all that we are destined to see, that the meaninglessness seen by the preacher is really the triviality of life on earth compared to that of heaven.

The preacher prides himself on his wisdom, but also says that with much knowledge comes much vexation. He wishes to give advice and share the wisdom in a series of proverbs but at the same time it is hard to take him seriously with him constantly reminding you that all of it is meaningless.
Finally he decides that it is better to enjoy your vain life than dwell on its vanity. Very confusing.