Headcanon- Merlin tells Arthur he loves him in acts of service, constant and reassuring service to him yet Arthur overlooks it because of how he’s been raised. Arthur tells Merlin he loves him in touch, Arthur has never acknowledged how touch starved he is but he’s constantly running his hand over Merlin’s arm or shoulder, just a fond ruffle of his head or a clap on the back. Merlin doesn’t acknowledge it because he’s always being touched. He grew up with affection.