This has been gnawing at my brain since I first heard it
because
And the way Arthur says it is so honest, so serious, so earnest. He's exhausted but he makes sure to look at Merlin's right in the eye. Like this comes from the bottom of his soul and he absolutely, desperately needs Merlin to understand (and considering he most likely knew he was going to die at this point, well). Look at Merlin's face. There's such solemnity and vulnerability in this scene. It hurts to watch.