From the ESV Study Bible‘s Introduction to Ecclesiastes (italics/bold mine):
One can see the Preacher’s most distinctive contribution from the way he uses the term “find out” (see note on 3:11). Every human being wants to find out and understand all the ways of God in the world, but he cannot, because he is not God. And yet the faithful do not despair but cling to God, who deserves their trust; they can leave it to him to make sense of it all, while they seek to learn what it means to “fear God and keep his commandments,” even when they cannot see what God is doing. This is true wisdom.
The history of salvation is the grand overarching story of the Bible; embracing it gives coherence to all of life. It calls each of God’s people to own the story, and it dignifies each one with a role in the further outworking of the story. Nevertheless it is impossible for any human being to fully grasp how his or her decisions will contribute to God’s grand scheme; and Ecclesiastes helps people to see that they do not have to understand this. Each of the faithful, by “fearing God and keeping his commandments” (12:13), participates in ways that he cannot “find out,” trusting that God will take care of the big plan.