Jesus refused to heal him.
		
		
	 
Because  of  the surpassing greatness of the revelations, for this reason, to  keep me  from exalting myself, there was given me a thorn [splinter] in  the  flesh, a messenger of Satan to torment meto keep me from exalting   myself! 
 Concerning  this I implored the Lord three times that it might leave me. And He has  said to me, My grace is sufficient for you, for power is perfected in  weakness.  
 Most gladly, therefore, I will rather boast about my weaknesses, so that the power of Christ may dwell in me.  
 Therefore  I  am well content with weaknesses, with insults, with distresses,   with persecutions, with difficulties, for Christs sake; for when I am   weak, then I am strong (2 Corinthians 12:7-10).
Matthew Henry Commentary 2 Corinthians 12:7-10
....2. The design of this was to keep the apostle humble: 
Lest he should be exalted above measure, 
2 Cor. 12:7. Paul himself knew he 
had not yet attained, neither was already perfect;  and yet he was in danger of being lifted up with pride. If God love us,  he will hide pride from us, and keep us from being exalted above  measure; and spiritual burdens are ordered, to cure spiritual pride.  This thorn in the flesh is said to be a messenger of Satan, which he did  not send with a good design, but on the contrary, with ill intentions,  to discourage the apostle (who had been so highly favoured of God) and  hinder him in his work. But God designed this for good, and he overruled  it for good, and made this messenger of Satan to be so far from being a  hindrance that it was a help to the apostle.
 3.  The apostle prayed earnestly to God for the removal of this sore  grievance. Note, Prayer is a salve for every sore, a remedy for every  malady; and when we are afflicted with thorns in the flesh we should  give ourselves to prayer. Therefore we are sometimes tempted that we may  learn to pray. The apostle 
besought the Lord thrice, that it might depart from him, 
2 Cor. 12:8.  Note, Though afflictions are sent for our spiritual benefit, yet we may  pray to God for the removal of them: we ought indeed to desire also  that they may reach the end for which they are designed. The apostle  prayed earnestly, and repeated his requests; he besought the Lord 
thrice,  that is, often. So that if an answer be not given to the first prayer,  nor to the second, we must hold on, and hold out, till we receive an  answer. Christ himself prayed to his Father thrice. As troubles are sent  to teach us to pray, so they are continued to teach us to continue  instant in prayer.
 4. We have an account of the  answer given to the apostles prayer, that, although the trouble was not  removed, yet an equivalent should be granted: 
My grace is sufficient for thee.  Note, (1.) Though God accepts the prayer of faith, yet he does not  always answer it in the letter; as he sometimes grants in wrath, so he  sometimes denies in love. (2.) When God does not remove our troubles and  temptations, yet, if he gives us grace sufficient for us, we have no  reason to complain, nor to say that he deals ill by us. It is a great  comfort to us, whatever thorns in the flesh we are pained with, that  Gods grace is sufficient for us. Grace signifies two things:[1.] The  good-will of God towards us, and this is enough to enlighten and enliven  us, sufficient to strengthen and comfort us, to support our souls and  cheer up our spirits, in all afflictions and distresses. [2.] The good  work of God in us, the grace we receive from the fulness that is in  Christ our head; and from him there shall be communicated that which is  suitable and seasonable, and sufficient for his members. Christ Jesus  understands our case, and knows our need, and will proportion the remedy  to our malady, and not only strengthen us, but glorify 
himself. His strength is made perfect in our weakness. Thus his grace is manifested and magnified; he ordains his praise out of the mouths of babes and sucklings.
 III. Here is the use which the apostle makes of this dispensation: 
He gloried in his infirmities (
2 Cor. 12:9), and took pleasure in them, 
2 Cor. 12:10.  He does not mean his sinful infirmities (those we have reason to be  ashamed of and grieved at), but he means his afflictions, his  reproaches, necessities, persecutions, and distresses for Christs sake,  
2 Cor. 12:10.  And the reason of his glory and joy on account of these things was  thisthey were fair opportunities for Christ to manifest the power and  sufficiency of his grace resting upon him, by which he had so much  experience of the strength of divine grace that he could say, 
When I am weak, then am I strong.  This is a Christian paradox: when we are weak in ourselves, then we are  strong in the grace of our Lord Jesus Christ; when we see ourselves  weak in ourselves, then we go out of ourselves to Christ, and are  qualified to receive strength from him, and experience most of the  supplies of divine strength and grace.