Why did Paul say he dared not share what he saw and heard?
Paul refrained because sharing what he saw and heard in that experience would cause people to fall at his feet and worship him. To prevent him from being exalted above measure, God gave him a thorn in the flesh as a constant reminder not to repeat those things.
Les explains that Paul was saying the same thing about his revelations as the idea of discovering Noah's Ark: if people could put their finger on it, they would beat a path to its door and worship it. Paul said if he were permitted to share what he saw and heard in that experience, people would fall at his feet trying to worship him, so he said, "I dare not." To keep him humble and prevent him from being exalted above measure through the abundance of the revelations, God gave him a thorn in the flesh, the messenger of Satan, as a constant pestering reminder: "Paul, don't you repeat these things. Don't you get puffed up. Don't you try to reveal to the world all that I've shown you." Les notes that God does not Himself inflict discomfort or pain; the thorn is called the messenger of Satan.