The Futility of life as it is characteristically lived out ‘under heaven’ is captured here in terms of people trying to straighten what is twisted and to count that which is ‘lacking.’ The key to understanding the first line is found in 7:13 ‘Consider what God has done: Who can straighten what he has made crooked?’ The emphasis there and in 7:14 is on accepting what comes from the hand of God rather than striving with and struggling against it.
Provan, I. (2001) Ecclesiastes/Song of Songs: The NIV Application Commentary, 77