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He's in a pub on Christmas Eve.
It's one of the most depressing places he's ever seen, with big burly men getting drunk in the corner, singing carols, and getting more and more hammered by the minute. It's the home of the family-less, with a cracked chalkboard announcing their turkey special, only two pounds.
At the bar, there's a few old men, sitting hunched over their scotch and soda, and a young one, who's pushing away his whiskey like it's disgusting him.
Sirius makes his way over to the bar, and sits down. He waves away the bartender, and ignores all the odd looks around him- Who is that? What the hell is he wearing? It looks like a glorified bathrobe. He's only got eyes for the tired-looking man next to him.
Without looking at him, Remus says, "Leave me alone. I know why you're here, Sirius. You don't feel the same way. It was a mistake telling you."
Remus looks tired, like he hasn't slept in days, and Sirius remembers with a jolt that he probably hasn't—remembers that this morning, before everything changed, he was at Remus's bringing him chocolate because the transformation had happened a day ago.
Sirius gets angry at this, like he does whenever he thinks about Moony's "furry little problem"—Why Remus? Why the one, who out of all of them, was so basically good? He takes Remus's arm, momentarily forgetting that just a second ago, he was fretting over how tired Remus looked, and drags him out of the pub, slapping some money down on the bar.
Remus looks at him, the streetlight reflecting off his hair, and as always, when Sirius looks at him he's struck by how much his heart hurts when he looks into Remus's eyes and sees all the pain he's had, all the suffering he's been through. He used to think that this was normal, that friends cared that much about their other friends. But when Remus told him that he was in love with him, Sirius realized that in eighteen years of being alive he's only felt like that about one person.
Remus looks at him bemusedly, and Sirius realizes that they've been standing there for almost a minute now and he hasn't said anything. He's not sure what he would say to convince Remus that he's not lying.
So he doesn't say anything, just grabs Remus by the back of the head and kisses him like he's never kissed any of his little distractions. And it feels more right than anything he's ever done, and in that moment it doesn't matter that there's a war going on, or that they could all die at any moment. His whole world is on that sidewalk in front of the Muggle bar, and it's a world of streetlights and wonder and chocolate and Remus. And that's all he needs.