"I’ve already thanked you for the kingdom we built together. You were always my warlock Merlin, whether I knew it or not, I am grateful for your magic. What I really need you to understand now is that I was always my happiest with you. From the very beginning when I was just Arthur the Prat and you were just Merlin the idiot. So no, I shan’t be leaving. I didn’t want to in my last lifetime so I won’t be now."
"…I only meant are you leaving to get the milk like you said you would hours ago. I just wanted a cup of tea Arthur…"
"Oh…Well erm yes, I’ll be back in a minute then"
"Before you go, I’m glad you’re not leaving because erm… ditto."
"…Ditto Merlin… Really? Ditto? Bloody Hell I might change my mind and leave after all… Oh don’t give me that look. Fine get your arse over here and mmpph!"
Or one of my headcanons part two: In which Arthur comes back from Avalon, moves in with Merlin and can see that Merlin is still a bit haunted by Arthur’s previous state of being dead and is worried that Arthur might disappear. Arthur in all his manliness created a little speech to give Merlin to reassure him and waited for an appropriate segue because he refused to bring up feelings first. Merlin being a bit dense to Arthur’s need never brought it up so Arthur had to take what he could get. Or Arthur and Merlin tell each other that they’re in love very awkwardly and some kissing happens at the end.
Apologies for the awful writing but I wanted to knock something up before I go away and I screencapped the dragon’s call, when they were all happy and cute. And I have promised some happiness for a while now so… yeah.