It doesn't matter what his specific 'thorn' was. What matters is the revelation from God.
No matter what we may be going through, there is a reason for it. Whether it's to show us that it's when we stop trying to do things on our own that God can finally do what He needs to do in our lives.
2 Corinthians 12:7-10 (KJV)
7 And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure. 8 For this thing I besought the Lord thrice, that it might depart from me. 9 And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. 10 Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong.
Peter also teaches on a similar note:
1 Peter 1:6-9 (KJV)
6 Wherein ye greatly rejoice, though now for a season, if need be, ye are in heaviness through manifold temptations: 7 That the trial of your faith, being much more precious than of gold that perisheth, though it be tried with fire, might be found unto praise and honour and glory at the appearing of Jesus Christ: 8 Whom having not seen, ye love; in whom, though now ye see him not, yet believing, ye rejoice with joy unspeakable and full of glory: 9 Receiving the end of your faith, even the salvation of your souls.
Whether Paul's thorn was full on demonic attacks, a person under demonic influence, or an actual physical ailment, the lesson we should take away - and the purpose for Paul writing of it - is that when we are in full subjugation to God is when He is able to do His greatest works in us. Whether it's teaching, healing, preaching, evangelizing, or whatever ministry God has given us. It's as John the Baptist said:
John 3:30-31 (KJV)
30 He must increase, but I must decrease. 31 He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all.
