Commentaries:
In chapter 2, Solomon launches into what he had learned about his works of building material things like houses and gardens and seeking even greater wealth. His conclusion? All of these material achievements were nothing but vanity, a grasping after wind.
He finds no real, sustained profit in them, nothing that truly added to his quality of life, no lasting fulfillment. He does not mean they resulted in no sense of achievement or passing pleasure, but that their fruit never truly fulfilled God's purpose for man. Therefore, those things are poor substitutes for a sustained sense of well-being. He then proceeds into an exploration of wine and entertainment. These are simply another form of materialism, ways of pleasing the flesh. He concludes that they, too, are folly, a mad pursuit.
John W. Ritenbaugh
Ecclesiastes and Christian Living (Part Seven): Contentment
The wise person knows where he is going. He has goals in his life and works hard to achieve them. His "eyes are in his head." The fool, conversely, seeks folly, mirth, pleasure, and so forth.
Solomon discovers that the stupid idiot shares the same fate that he, a wise man, does: They are both going to die. He asks, "What good is it, then, to become wise?"
John W. Ritenbaugh
Ecclesiastes and the Feast of Tabernacles (Part 2)