Why did the Lord appear to Paul over and over?
The Lord appeared to Paul over and over to give him encouragement in spite of all the sufferings and disappointments he had to go through. This also demonstrated that Paul had direct contact with Christ, defending his authority as the God-ordained apostle of the Gentiles.
Les explains that Paul constantly had to remind the Corinthians and us that he was the God-ordained apostle of the Gentiles. Even the mostly Gentile Corinthians accused him of lacking authority compared to Peter, James, and John, who had been with Christ for three years. Paul shows he also had direct contact with Christ, not by walking Palestine's dusty roads, but the Lord appeared to him over and over, especially to give encouragement despite his sufferings and disappointments. In verse 22 he notes the twelve had nothing over him in genealogy—Hebrew, Israelite, seed of Abraham—and he then moves to ministers of Christ.