I have no scriptural basis for this thought whatsoever, but I wonder...?
Does everyone have a "thorn in the flesh" that they are left with just to keep them humble? Bi-polar, alcoholism, depression, etc ad infinitum...
Some of our greatest Christian souls have been plagued by these and more. These are the ones who know God's grace because they're so screwed up. Those that can't admit their faults are those who can't understand grace.