"I know you will be a great king." There was no doubt. For all Arthur's faults, he cared about his people and was a good man at heart. Even if they couldn't get along all the time, Merlin didn't question that Arthur would be a good king in the future.
"I plan on being at your side then too, arguing with you about something new I'm sure." He teased. Merlin couldn't say he was surprised if that was the root of Arthur's fears, not being a good king. A lot was expected of him. Not that it would be difficult to be good when one had to follow Uther. Uther could make anyone look like a saint in comparison.