There are limits to everything. Ciel knows this.
Geometrical figures are defined by their limits, their outlines. So is the sea. So is the sky.
And everything has a limit.
Loyalty, pride, revenge even. Humans. Demons.
Sebastian's patience and his willingness to trade time and quantity for quality. For Ciel's soul.
But it is such fun to tease the damned, eldritch creature; such fun for Ciel to push boundaries until he is toeing the line, balancing on the edge of the cliff with the wind blowing from behind.
Ciel shouldn't be so reckless. So... hedonistic, perhaps. It isn't like him, doesn't resemble his usual cold and calculating nature and yet-
oh, Sebastian's unreadable smile and his ever so subtly snide remarks and his inhuman, glowing eyes
-how irresistibly exhilarating it feels to step off of the ledge.