Something I love a lot about sebaciel's relationship development is the way BoA is Ciel realizing it's possible for him to lose Sebastian, and the Germany arc is Sebastian realizing it's possible to lose Ciel.

Like, BoA's a little more obvious? Black Butler relies heavily on how OP Sebastian is, and Ciel's always treated him as his trump card. So Sebastian nearly dying, swatted away as easily as a fly from someone they know, is obviously a shock to his system.

Germany maybe requires a bit more explaining. Of course, in theory, Sebastian knew Ciel could be killed—duh, he's the squishy mortal! And Sebastian has a much clearer idea of his own powers as well, so he knows the limits of what he can protect Ciel from. Except when he doesn't

Part of it, yes, is the physical attack, one that takes Sebastian by surprise and that he is genuinely powerless to save Ciel from—it's only thanks to Sieglinde Ciel survives. That's not something Sebastian's used to. But the much bigger issue is the mental regression it invokes

Sebastian didn't just nearly lose Ciel physically—something he was fully aware was possible and is largely able to prevent—he DID lose him mentally, for a hot minute there. And there was literally nothing he could do to reverse the damage.

And the nail in the coffin was Sebastian facing the only thing he had left to do—nullify the contract, and eat Ciel's soul. And even when that doesn't go through, Sebastian has now had to face how it'll feel when it inevitably happens for real.

Both arcs confronted them with the other's very real limitations, the very real risks they face, and in Germany's case, the very real finitude of the contract. They serve as a grounding reality, and a raising of stakes, and the best kind of character development there is.