I came to hate all my hard work here on earth, for I must leave to others everything I have earned. And who can tell whether my successors will be wise or foolish? Yet they will control everything I have gained by my skill and hard work under the sun. How meaningless! So I gave up in despair, questioning the value of all my hard work in this world. - Ecclesiastes 2:18-20 NLT
The Teacher has sought solace in work. Now, let's be clear about this, work is a good thing. Work is a holy thing because God did it. This book is not telling you that work is bad because it's not. What is bad is finding your life's purpose only in work.
The Teacher has worked hard. He's trying to create meaning and purpose in life from work alone, and now that he's had his work accomplishments, he's reflecting on how meaningless they are. After all his hard work, one day, he will die. After he's dead, someone else will take over what he's left behind. And the same is true for us.
One day, we will die. Perhaps we'll leave a company or organization behind. Maybe, like me, you'll leave a classroom behind. And we don't know if our successors will be wise enough to manage the work or foolish enough to banish working and responsibility. When work is your only purpose, that is a scary thought. When you work out of your purpose, that is a freeing thought.
If your only purpose is to work, then there is nothing to leave behind and certainly nothing you can take with you. But if you find purpose in your work, you can begin to leave a legacy behind.
Why was work meaningless? Because work by itself can't give you God's eternal purpose for you.