Ecclesiastes 2:23 (ESV)

23 For all his days are full of sorrow, and his work is a vexation. Even in the night his heart does not rest. This also is vanity.

That brings sadness and tension into your life. You keep finding out that even your work cannot give you that wonderful, happy life. All your work cannot provide the real meaning of your life. This is what Ecclesiastes makes clear in Ecclesiastes 2:22–23. To live for your work only is also in vain.