2 Corinthians 12:7, So, to keep me from becoming conceited because of the surpassing greatness of the revelations, a thorn was given me in the flesh, a messenger of Satan to harass me, to keep me from becoming conceited.
At times, God may permit a thorn in your flesh and allow Satan to harass you. Luther: “[Paul] had to cling to the comfort of God’s words that he should be satisfied with His grace [v. 9] and through it overcome his weakness” (Church Postil, sermon for Epiphany 1 on Luke 2:41-42, LW 76:198). Thus, thorns are permitted in our flesh so we may learn to always cling to God’s grace and not lean on our own ability and understanding (Proverbs 3:5-8).
What we may see as unfairness in this regard is actually God’s grace, for He is not content to let us walk down a dark path into despair. Whatever your thorn is—addiction, anger, laziness, illness, disease, etc.—continually pray that God remove it, and when He does not, lean on His grace—that is, pray ever more that He provide the ability to overcome your weakness and to fall back on Him when you fail.
The Lutheran Column 