2 Corinthians 12:9 (ESV) … But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly of my weaknesses, so that the power of Christ may rest upon me.
After three requests and three denials, God directly spoke to Paul: “My grace is sufficient for you, for my power is made perfect in weakness” (12:9a). God wanted the apostle to keep the thorn. It was one way by which he could learn to trust God more. The grace of God not only saves, it also sustains. It is abundant and available for the daily needs of believers (John 1:16). The strength of God achieves its goal or is finalized through human weakness. This is true of salvation: “For when we were yet without strength, in due time Christ died for the ungodly” (Rom. 5:6). It is also true of service. When a believer is physically weak, they must depend upon the ability of God to accomplish his tasks.
When Paul learned the truth of that divine revelation, he changed his attitude regarding his affliction from prayer to praise (12:9b). He stopped asking God to remove the thorn and, instead, he thanked Him for it. He did not just endure the ailment, but he “gladly” gloried in it. Paul wanted to be enveloped by the ability of God. If it meant that he must endure infirmities to attain such power, then he was willing that the infirmities remain.
Paul was not asking anyone to feel sorry for him just because he suffered so much. Rather, he “took pleasure” (eudokō) in all of the hardships that he endured “for Christ’s sake.”
The reason for such spiritual pleasure is paradoxical: “for when I am weak, then am I strong.” He was not strong in his own strength, but in divine strength. The reverse is doubtless true. When a believer is physically strong, they may not rely upon God as much and thus become spiritually weak. Spiritual, inner power is far more important than physical, outward health.
 Gromacki, R. (2002). Stand firm in the faith: an exposition of II Corinthians (pp. 205–206). The Woodlands, TX: Kress Christian Publications.