During this sequence that’s supposed to feel victorious and like, “yeah things aren’t perfect, but the good guys won today!” Merlin watches Guinevere and Arthur hug intimately and you know what? He looks sad.
He looks like he wishes he could touch Arthur so freely and affectionately. He looks like he’s desperate to be seen for everything he is and everything he’s done for him. He looks like he’s accepting he’ll never be with Arthur like that and it’s breaking his heart.
That man is deeply in love and he can’t do anything about it. He is happy for them because he just wants them to be happy, but that doesn’t mean it doesn’t feel like a dagger in his heart.
It reminds me of that quote from Sherlock, “you look sad when you think he can’t see you”