Ecclesiastes' observations about work and wisdom and pleasure in a meaningless, mortal and twisted world under the sun are true for ministry too, from a human point of view..
Under the sun ministry, too, is meaningless toil. One man's ministry is a deadend, plateaued numbers, minimal conversion. Another man comes along, does the same thing, and yet is massively blessed. This is a miserable business.
One man gives his life to God, makes great sacrifices for the gospel and his wife is diagnosed with cancer and dies, leaving him with four kids to look after. Another man is corrupt and selfish and yet is making millions of dollars a year and has a happy family.
Meaningless! Meaningless! says the teacher. Utterly meaningless!
This is why we need to fix our eyes not on what is see, but what is unseen. For what is seen is temporary, but what is unseen is eternal.