This passage is also in the middle of Solomon's pondering and shouldn't be taken verbatim - much like most of Job is bad advice from his friends. Let's look in the context:
14 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.
15 Whatever is has already been,
and what will be has been before;
and God will call the past to account.
I think this shows that God's work is much more important than man's toils. God's work is eternal, and man's work means nothing.
16 And I saw something else under the sun:
In the place of judgment—wickedness was there,
in the place of justice—wickedness was there.
17 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."
This shows that even in the place of rule, the government, there is evil. In justice and in judgment, wickedness was there. Everything that has a season will be judged still. A time for the killing and a time for its judgment. A time for healing and a time for its judgment.
18 I also thought, "As for men, God tests them so that they may see that they are like the animals. 19 Man's fate is like that of the animals; the same fate awaits them both: As one dies, so dies the other. All have the same breath; man has no advantage over the animal. Everything is meaningless. 20 All go to the same place; all come from dust, and to dust all return. 21 Who knows if the spirit of man rises upward and if the spirit of the animal goes down into the earth?"
These seasons show how we act like animals? Does an animal kill or does an animal heal? Does the animal war or does the animal live in peace? This passage also shows how this text isn't to be taken straight and Solomon is expressing doubt in Man's soul after death.
I guess to finish this off would be the same way Solomon finished the Ecclesiastes with Chapter 12:
13 Now all has been heard;
here is the conclusion of the matter:
Fear God and keep his commandments,
for this is the whole duty of man.
14 For God will bring every deed into judgment,
including every hidden thing,
whether it is good or evil.