'Toiled Under the Sun' in the Bible

So I hated all the fruit (gain) of my labor for which I had labored under the sun, because I must leave it to the man who will succeed me.

So {I began to despair} of all the toil with which I toiled under the sun.

For what does a man get from all his labor and from the striving and sorrow of his heart with which he labors under the sun?