It was alright that Sirius didn't believe it. That was half of why James was so stubborn about believing it, to be perfectly honest; someone needed to be optimistic, to keep them all from being dragged down by hopelessness and despair. Sirius and Lily had had quite enough of living in fear and confusion, and the process of coming out of it was hard, but it was for the better. There was so much to be gained, and (he hoped) very little to be lost.
"For what?" James asked. He knew about what had happened with Remus, vaguely; he knew that something had happened while they were younger, but he didn't know the details. Dealing with Sirius was very complicated for Remus, far more complicated than it was for James. But he hadn't noticed anything happening with Lily.
He lowered his hands, but shifted over to bump one of his shoulders companionably against Sirius's instead. Things were complicated between them, too, but the only thing James really wanted and needed was to be friends again, with whoever Sirius was now. There wasn't anything they couldn't sort out, if they could figure out how to get that back.