….::leave your gloves at the doorstep.
They play a game of masters and servants, of walking on the line between complete trust and betrayal. Ciel feels that he needs to be on guard at any time of day and night but he also feels that nights are when it is impossible to be in check.
Sebastian does it because he is the sin itself; because he could not deny tainting the soul he had been promised. Ciel sees this expectation in his butler's eyes; it is unmistakable (or rather, he suspects it – there is no way to understand Sebastian's eyes) and dangerous.
Ciel does not like to waste his time thinking why he does it himself; the reason why he lets his butler take off his gloves at night
(or the truth behind the question: who is it that really allows the other?).
