Written for a cookie challenge, before HBP came out: how Remus' faith survived so long, and how it finally died.
He thought many years later their accepting what he was began that change, taught him drama and excitement could happen with a happy ending. He couldn't track his change of attitude any further than that. All he knew was when James suggested a bit before one full moon in their fifth year that they didn't have to necessarily stay in the Shrieking Shack the entire night, the thought of a wild adventure caused a warm glow in his heart. His life turned from a horror story to a romantic tale like those they read as children, all through his friends' generosity and love.
In the long run, he thought Sirius's nearly killing Snape only ended up strengthening this belief. To be sure, his first thought was that he had lost it, that his philosophy as a first-year would return, because normal boys didn't nearly murder people. But then his friendship with Sirius was repaired, then deepened, and if he thought romance was a darker thing, because it could involve such wicked deeds, he also thought love and the moral goodness of a person would triumph, no matter what.
He thought the world a much darker thing after James and Lily's death. But he didn't stop believing in the existence of true, noble, untainted heroism. He'd seen it in Peter, after all. And he didn't even stop believing in the unbreakability of love, because he didn't stop loving Sirius, no matter how much he hated the man also. He had never really believed himself to be any sort of martyr, despite his being a werewolf, but he was willing to allow his heart to martyr itself for the love of a dark wizard.
Perhaps there were dark moments, when he believed Sirius had not loved him, or even had never loved him in the first place, but somehow, he could never believe that for more than moments at a time. Things between them has been rough at the end, seeded with obvious suspicion on both sides, but Remus had needed to only look into Sirius' often agonized face to know Sirius loved him.
And it really wasn't as much of a conflict as one might think. After all, werewolves had a reputation for being dark creatures through and through, even when it wasn't the full moon, and from the beginning Remus had known that Sirius Black saw the world as he wanted to see it. That he could believe Remus willing to change sides for him was very far from impossible.
His belief eventually materialized itself into a reoccurring dream. There Sirius would tell Remus of his support of Voldemort, and try to convince Remus to join him. Promises of vows like those of marriage always accompanied Sirius' speech. But then Remus would reject him, feeling his heart break in two as he did so, but knowing he was making the right decision. They would come to blows then, of course, but they would both hate it more than anything.
Sirius had always loved that which was darkly romantic, and much more than anything of more conventional romance.
But in the end the truth had come, and then Remus stopped believing in heros. Peter was not a hero, instead, he was a coward who had sold them all out not even out of misguided beliefs, as Sirius would have, but out of self-preservation. Much more mundane. Intellectually, Remus supposed, he'd always known of the existence of such people. But at some point he had refused to face that fact, and facing it now put an end to most of his beliefs.
And it was two years after that when he couldn't believe in a happy ending anymore either, or even a satisfactory one. If Sirius had died with his name clear, that at least would be something to be glad of. Or even if he'd died with a purpose, such as shielding Harry from a fatal curse. But the outcome of that evening at the Department of Mysteries would have been exactly the same if Sirius had not gone through the veil.
And to top it all off, Remus was himself a coward. Because even though he knew Harry himself was growing dangerously romantic, too heroic, dooming himself to be destroyed by the world as Remus, Sirius, and James had been, Remus could not bring himself to say a word to stop him.