Sugar Into Salt

he never suspected that ciel could be harmed while being in his presence, which is why he took him to investigate a forest people believed to be cursed. in the same way, he didn’t think anything could happen to ciel at the music hall. as he himself said, “i failed to take this situation seriously because i assumed i would be able to sense immediately if you had been harmed or wounded.”

obviously, the situations in the book of circus and book of atlantic arcs won’t happen again, but the way he panicked after ciel "fell ill" following their visit to the forest and after catching the scent of his blood is another step in how increasingly protective sebastian becomes toward him. it’s another lesson for him—another mistake that can never be repeated. because every time he starts believing ciel is safe as long as he’s by his side, someone proves to him that he can never be quite so certain.