[The Ciel he knows certainly would not mean that sincerely - in most cases, wouldn't put so much effort into making it sound sincere. Definitely not when talking to Sebastian. He doesn't even stop to consider the possibility that this is some kind of test, or a game - not with that distinct lack of misery imprinted on Ciel's soul.
How dare someone put their filthy, meddling hands on what belongs to Sebastian. As Ciel turns and walks away from him, Sebastian remains where he is, smile still in place, seething. His instinctual urges of how to act... well. Most likely, he ought to wait. Think it over. Have restraint.
Then Ciel turns to address him again, and Sebastian's smile widens.] Ah.
You see, I serve in the Phantomhive household. [And he bows, deeply and properly with his hand over his heart.] My name is Sebastian Michaelis.