"Right," Sirius said, more for something to say in response, although the word sounded lost to him, as if he had didn't know what to say. And he didn't. There probably wasn't anything else to say. Remus was leaving and that was the end of it. At some in time, in a very different place, they would meet up again, but all these moments would be lost -- presumably to both of them, but definitely to Sirius.
And he didn't precisely know if it was because he was feeling brave or scared that he squeezed Remus' hand a little more soundly. Without letting himself think about it too much, he tugged Remus abruptly closer as he slid across the floor, letting his lips gently brush against Remus'. Because even if this was the end and even if it was a lost moment, Sirius needed it to mean something -- needed Remus to understand how much he cared and that he wasn't abandoning their future by staying here.