The modern-day descendants of Israel seem to exhibit an especially high degree of idealism and perfectionism. These are not inherently bad traits, because God indeed requires us to strive to be perfect and to live according to His ideals. Sometimes, though, we can create stress for ourselves when we have expectations of perfection because, as Solomon teaches, our world is not perfect.
God has blessed the nations of Israel tremendously, and with those blessings comes the ability to overcome many obstacles. Yet there are aspects of our surroundings that are simply broken—words cannot be unsaid, deeds cannot be undone, and crookedness cannot be straightened.
This axiom in Ecclesiastes 1:15 is connected to the previous verse, which speaks of “all the works that are done under the sun,” giving verse 15 its context. All the works of man—everything in this kosmos, this world apart from God—include a crookedness that cannot be rectified. The number of things lacking in all of man's works is so great as to be uncountable.
By way of definition, the Hebrew word translated as “crooked,” 'avath (Strong's #5791), is used less than a dozen times in the rest of the Old Testament. Its basic meaning is “to wrest,” which is “to forcibly pull something from a person's grasp” or “to obtain by wrenching with violent, twisting movements.” In essence, it is the assertion of one person's will against another's, and the result is damage that can never truly be repaired.
In other places, 'avath is linked with the perversion of justice (Job 8:3; 34:12). It can mean wronging someone or dealing perversely with someone (Psalm 119:78). It indicates turning things upside down or upsetting the natural order of things (Job 19:6; Psalm 146:9). Finally, it can refer to subverting someone in his cause and falsifying the scales (Lamentations 3:36; Amos 8:5).
Solomon is saying that, once the natural order of things has been upset by this willfulness, it is essentially impossible to make those things right again. The order of things cannot be equalized (which is what the word translated as “straight” means), even though there may be a salve that can be applied. When something has been wrested from another—when one person's will has been asserted at the expense of someone else's—it sets things into motion that cannot be equalized. A measure of crookedness will always remain in man's works.
Thus, because of human nature and willfulness, anywhere we find human actions, we also find disorder and incompleteness. We see irregularity and deficiency. Not only that, but we also discover mankind's utter inability to truly fix them or fill in what is lacking.