Disclaimer: They belong, angst and all, to J K Rowling...I just like rubbing the salt in, I suppose.
No Happy Ending
Werewolves don't get happy endings. All chance of that disappears the moment you're bitten, and anything that follows is only putting off the inevitable. Remus feels no surprise at Sirius' fall; shock, yes, grief most certainly, but no surprise. He already knew the last two years were so much more than too good to be true.
He sinks back into routine, unable to show his devastation as those around him weep for his fallen lover; and if he spends more time shut in his room, nobody remarks upon it.
There is very little of Sirius about Grimmauld Place; it was only his house, never his home. Sirius was not simply chafing at his captivity, he was burning inside at being stuck in that place, the house he had always hated. The illustrious House of Black, come down to him only because he was the last one left. Remus did once allow himself to hiss, "Are you happy now?" at the shrouded portrait of Sirius' mother, but it gave him no response and left him feeling strangely petty.
Remus spends as much time as possible in his room, losing himself in memories of happier times; a happy ending may not be his lot, but he had many moments of happiness nevertheless, only slightly tainted by the certainty that they could not last.
His first day at Hogwarts, the immaculately groomed black-haired boy who took him under his wing on the train; Sirius' first act on leaving the station had been to remove the smartening charm his mother had forced upon his unruly hair, and for the rest of the time that he knew him, Remus only saw Sirius looking smart a handful of times. New friendships forged on that first train journey, and the dearest of all had been Sirius.
That day some time in the fourth year, when Remus looked across the common room at Sirius, sitting by the window, and saw not his dear friend but someone else entirely, someone enchanting and mysterious and utterly wonderful.
The two years of deliciously suppressed wonderings, does he like me too, will I ever have the courage to tell him, what will he say if I do? The warm glow inside when his friends became animagi for him; nobody had ever done something so selfless and wonderful just for him.
And oh, that night in the sixth year, sitting up late in the common room doing a particularly knotty Potions essay, when Sirius had appeared as if from out of nowhere and perched himself on the arm of Remus' chair; he had slipped an arm around Remus' narrow shoulders and leaned down so that their eyes were level. "You know, Moony," he had said, "you really are utterly gorgeous." And he had tilted Remus' chin up with one finger, and he had kissed him gently but soundly, and Remus had finally stopped holding on and had allowed himself to fall, completely and irredeemably.
Short years of stolen moments and secrets well-kept, marred only at the end by mistrust and doubts, and then the catastrophic realisation that perhaps he had not known Sirius as well as he had thought.
Remus had known all along that it would end, but he had found it harder to be stoic at the age of twenty-two; he had not yet had those long years of practice.
And then hope, light in the darkness, an invitation to teach at Hogwarts, and for a few months Remus was happy again. Harry and his friends were a joy to behold, and Remus knew that all the sacrifices were worth it. And it was crowned, one fateful night in the place of his old captivity, by the return of Sirius, and the discovery that he was no traitor after all; but of course with happiness comes despair, and the wolf conspired to ruin everything he had worked so hard for. He left his childhood home, eaten away by guilt and remorse. And yet Sirius came to him, found him and forgave him for everything; and for a while, happiness was theirs once again.
Remus is not ungrateful. He knows he has had more happiness than a werewolf deserves, and that is enough for him. He will learn to deal with his loss, put it away and continue with his life. There is no happily ever after, but what of it? That, after all, would have been too much to ask.