y’know, Merlin didn’t seem surprised at all when Arthur hugged him like that in 4x06. maybe it was because of the spell, but…Arthur is tactile. he shows his affection through physical contact. so it makes sense that he’d hug his best friend more than a couple times over all the years they’ve spent together, especially when they spend half that time saving each other’s lives.
basically what i’m saying is think of Arthur hugging Merlin like 200x more often than they showed