"I know that there is nothing better for men than to be happy and do good while they live. That everyone may eat and drink, and find satisfaction in all his toil - this is the gift of God." NIV translation
Our season on this earth should be a time of happiness, if we are in Christ. God has prepared things for each of us to do and, if we listen to the Spirit and follow, He will lead us to do good things and to enjoy our walk with Him. This having God with us to guide and comfort us comes through His grace (a gift).
"I know that everything God does will endure forever; nothing can be added to it and nothing taken from it. God does it so that men will revere him." NIV translation
Everything that was or is was put in place by God and, though it is hard for us to understand, we cannot add or take away anything from what He has already done. What is the purpose of everything? Quite simply, all of creation was meant to worship God. The word "revere", here, is the Hebrew word "yare" which means "to fear" but not in the sense that we think of it. In Hebrew, the thought is connected to love and not terror. It is a behavior and not a feeling. The behavior that is described is worship.
"Whatever is has already been, and what will be has been before; and God will call the past to account." NIV translation
God is a god of patterns because of the fact that He is unchanging. He does not need to change because He is perfect in all of His ways. I have heard many Christians ask the question: Why should I study the Old Testament? This verse is the best reason that I can give them and I have to ask them the question: Do you want to know what God is going to do? If their answer is yes, then, we must study the past (Old Testament) because God is a god of patterns.
"And I saw something else under the sun: In the place of judgment - wickedness was there, in the place of justice - wickedness was there. I thought in my heart, 'God will bring to judgment both the righteous and the wicked, for there will be a time for every activity, a time for every deed.'" NIV translation
Does it seem like evil is overtaking the world? That is what Solomon sees as he compares justice here on the earth with the justice that is to come through Jesus Christ. He saw the coming of two judgments as the actions of the sons of God will be examined and rewards will be given while the wicked will be judged and punished. This is what is spoken of by John in Revelation 20.