In the preceding chapter Paul pointed to his suffering as his authentication as a genuine apostle. In this chapter he appeals to his surpassing revelation and subsequent humbling and grace from God. These things would never make a resume' today.
The 'thorn' was a messenger from Satan. The messenger of Satan was an agent of God. God used it to give grace and strength. Why does Satan even try if God turns his best efforts into good?
Strength is made perfect in weakness. He boasts in weakness because of the opportunity it presents for God's strength to be obvious.
His good motives and good decision in sending Titus is a further proof of his apostleship. The church had not properly grieved or repented of their sin -- always a problem.
He views them as his children. He would gladly spend and be spent for them.