Chapter 5 has key verses that tell us what things we should do while here on earth whether we understand them or not. In other words,
God is God and is in heaven, we are down here. Who are "we" to question God. When or if God gives us any wealth and posessions, and enables us to enjoy them, then we should be content with what we have and be good stewards of it. Because in the end vs. 15 reminds us that "Naked a man comes from his mother's womb, and as he comes, so he departs." The blessings and possessions we have while on earth are gifts from God, we do not own them, for if it weren't for God, we wouldn't have anything. We need to "guard our steps" and be careful not to make a vow to God and not fulfill it. Work hard, enjoy it, but remember to thank the GIVER! vs. 12. The sleep of a laborer is sweet, whether he eats little or much, but "the abundance of a rich man permits him no sleep."
Consider this: God does not mean for us to understand everything about our life or his purpose for us. Instead, focus on our eternity and not on the mundane things in life. Your interpretation of the verses in Ecclesiastes may be very different from mine. That's ok, and the way it should be. God speaks through his word "differently" to each one of us. We need only to listen and God will speak.