Arthur's life is complicated. It's filled with politics and expectations and so many obligations that it makes him want to bash his head against the wall. Arthur's life is the life of a prince. Arthur's life is difficult, crushingly so.
Then, there's Merlin. Merlin with his easy smile, with his easy life that's hampered only by the whims of the prince he serves. Merlin whose most complicated problem is maneuvering the stairs while carrying armor.
Sometimes, when the nights stretch long with decisions to be made, Arthur stares into the fire and wishes himself into the life of his servant.