Paul asked God to remove a struggle he had in his life three different times. The Bible says it was a messenger from Satan. God didn’t remove it because He said His grace was so powerful that Paul’s struggle would show God’s perfect strength through  it.

2 Corinthians 12

5 That experience is worth boasting about, but I’m not going to do it. I will boast only about my weaknesses. 6 If I wanted to boast, I would be no fool in doing so, because I would be telling the truth. But I won’t do it, because I don’t want anyone to give me credit beyond what they can see in my life or hear in my message, 7 even though I have received such wonderful revelations from God. So to keep me from becoming proud, I was given a thorn in my flesh, a messenger from Satan to torment me and keep me from becoming proud.

8 Three different times I begged the Lord to take it away. 9 Each time he said, “My grace is all you need. My power works best in weakness.” So now I am glad to boast about my weaknesses, so that the power of Christ can work through me. 10 That’s why I take pleasure in my weaknesses, and in the insults, hardships, persecutions, and troubles that I suffer for Christ. For when I am weak, then I am strong.