I have always imagined it the incessant and irritating daily reminder of souls. We often picture a glorified splinter, which reminds Paul, daily, that he is to be weak that God may bestow true power. But there has to be much more to this thorn.
If it were only a glorified splinter, Paul would be a big baby to write in 2 Corinthians 12, "Three times I pleaded with the Lord to take it away from me." Come on, would a big splinter be worthy of such pleading?
In those times, "thorns" were used as a military device, and it was certainly much more than a splinter. It was much more like a big stake. They would pound these stakes (not much bigger than a thick tent stake...but sharper and more jagged) into the ground all over the place in an open area they were retreating through. This way, when the enemy came running through that field after them, they would be slowed down. Big stakes protruding from the ground would slow any army down.
Now re-imagine the thorn in the flesh. It takes on a little more intensity now. So why would Jesus want to give Paul a thorn? Well Paul was an amazing man who's testimony has been a root for the Christian faith. He wrote most of the New Testament. Now with accomplishments like that, can you imagine the arrogance he is capable of? But he wasn't arrogant. In fact, he was incredibly humble and vulnerable in his ministry. I think it is for this reason he was given a thorn that so tormented him.
His torment brought Paul to this kind of humility that reached millions. This thorn made him FEEL so weak that he could only depend on God. We know this thorn cut deep into Paul, but was never removed. It remained to continually bring Paul to brokenness and vulnerability, but it is this brokenness which forced Paul to rely so heavily on God. Not by our strength; but God's.
The Message calls "the thorn in the flesh" the "gift of hardship". I think it really can be anything for us. We do not know exactly what Paul's "thorn" actually was, but we know what it did. I think our "thorn" could be anything, as long as it does one thing...brings us to brokenness and vulnerability before a powerful God.
I wonder what my thorn...my gift of hardship may be. Peter Scazzero makes a list: "What might the 'gift of hardship' God has given you be? A child with special needs? A struggle with an addictive behavior that forces you to be vigilant every day and attend meetings regularly? Emotional fragileness with a tendency to depression, anxiety, severe isolation, or loneliness as a single person or widow? Scars on your soul from an abusive past? Childhood patterns...a physical disability? Cancer? Real temptations to anger, hate, resentments, bitterness, lust, pornography, or judgmental?"