I haven’t watched Merlin past about episode 10, season 1.
But this seems to me like Arthur knows about Merlin’s magic, but he’s in constant denial about it. So he doesn’t want to be told about it by anyone because then he’d have to face it. He’d have to do something about it. And maybe he doesn’t want to because he cares about Merlin a lot more than anyone [in the show] realises.
NON-FAN HEAD CANON ACCEPTED
WHY DID NONE OF US THINK OF THAT
Sometimes you need an outside eye to see such things.
“I was born with a stain. A mark. Like the mark of Cain. But is the mark of my father, my family. The mark of Pendragon. I have tried to be other than I am. And I have failed.”