@undercoverslime pointed out the inherent possessiveness of the contract mark: it's privatising Ciel's face, making the most obvious element of Ciel's physical personhood something he is obliged to hide from others - the eye patch adds an immediate physical removal and mystery to others - that can only be shown to Sebastian. If the face is the foremost visual point of relation and definition of personhood, then Sebastian's placing of the contract symbol obligates that personhood to be hidden and constructed as a facade to others, and further literalises the belonging of Ciel's true self to him - and his vice versa: his hands, the elements with which he most literally serves Ciel (and upon which Yana places a lot of symbolic weight) are shown to Ciel alone, the attributes of his facade are bared to his viewing only - beast and butler both. As @kuroposting puts it, it's a further construction of them as a conspiratorial in group, in which honesty about their true selves is beholder only to the other through the physical construction of their dynamic, forcing the public body into a privacy exposed only to the other. It's a very fundamental possession over a sort of psychological relation to the body (both one's own, and others towards one's body), that removes it from others' interaction and forces one to relate to the body in the context of the dynamic.
And the mark was placed at the outset, before their relationship came to develop to the intense extent it has. Between Sebastian's devoted aesthetics, and the potentially metaphorical (metaphysical) element of his hunger (a ravenousness dually literally and spiritual), and Yana's comments about a soul's taste being imparted to it by the hungering for it rather than necessarily being inherent, I can't help but wonder if Sebastian was looking for someone to get obsessed with from the outset, to quite literally fill the void. It's intriguing to think of Ciel not merely as an exemplary figure of interest who draws that out of Sebastian, but one whose embodiment of that quality is at least partially due to Sebastian's perception of him (or desire to percieve him) as being such. Which in turn bears relevance as to whether Sebastian came on a whim or not, and thus the responsibility Ciel would potentially feel as to the consumption of his brother's soul (something he possibly feels even more conflicted about given the composition of bizarre dolls) - their dynamic is comprised of as many fundamental misunderstandings as it is a shared, conspiratorial identity - in a sense that true, central, shared reality and mutual identity is unknowable to themselves. In a story concerned with identity, that implies a forthcoming mutual confrontation and acknowledgement of that.
There's a bemusing tragedy to the pair of them, but by now less immediately because of the predatory and coercive and carnivorous elements than for the way the dynamic they try and fail to have sits within that unresolved initial context.