He was in love, all right, with Remus Lupin which wasn't so unusual considering how long they'd known each other, but it was surprising because if Sirius expected to fall for anyone he figured it would have been James. They were best friends, for one thing, and James knew him better than anyone, better than Remus and much better than Peter. That was an attractive thing, when someone really knows you, and James wasn't bad-looking so when Sirius realised that he wasn't, in fact, in love with James he was utterly confused.
When Sirius was confused, he usually went to Remus about it. James wasn't much interested in serious discussions, and Peter was useless when it came to anything important, because he wanted to be like James and if James didn't do something, Peter wasn't likely to either. Which left Remus, and that was a good thing because Remus was smart, and sensible about things, and never failed to tell Sirius when he was being stupid - though more often than not it was well after the fact. Remus wasn't the most forward person in the world.
Sirius knew that. He sat at his desk in their empty dormitory - James was with Lily, and Peter and Remus were studying in the library - and twirled an eagle quill in his fingers. In front of him he had a square of parchment that had the name "Moony" scrawled across the top, and then nothing else. Sirius couldn't think of what to write, to tell Remus that he needed to talk to him about something very important. He couldn't tell him what exactly it was, because then Remus would wait days before he came to him about it, and Sirius didn't want to wait that long. He wanted Remus to help him figure it out right now.
Then again, Remus was always telling him to be more honest about things, and to get to the point, stop babbling. Sirius didn't know why he always babbled when he talked to Remus, though now he sort of suspected that being in love with him was part of the problem.
He licked the nib of the quill, and wrote:
Moony, I have a problem. It seems I am not in love with James. Which is weird, because I think I should be, but instead I'm in love with you. Isn't that the oddest thing? Anyway, I'm not quite sure what to do about it, and I was hoping you could tell me. I'm up in the dormitory, so come and see me when you get this.
Love, 
Padfoot.
He sealed it, and wrote FOR MOONY: STRICTLY CONFIDENTIAL (THIS MEANS YOU!) on the outside, just in case. Then he nickered softly for his owl, who fluttered down from her bedpost perch and nipped affectionately at his hands as he tied the note to her leg. "It's for Remus," he told her, "he's in the library. Try not to leave any droppings this time, or Madame Pince will make a book cover out of my hide."
The owl hooted and flew off out of the window. Sirius waited. He stood and paced around the room, gnawing at a jagged fingernail, wiping his fingers on his jeans. He made his bed and then James's. Peter's was already made, not that he'd ever go near it without dragonhide gloves on.
Twenty minutes later, after he'd picked up all his clothes, repacked his trunk and read a chapter of Transfiguration Today, Sirius leaned against his desk and tried of something else to do to pass the time when the door flew open and Remus was suddenly there. From the state of his hair and the fact that he was breathing rather loudly in the quiet room, Sirius thought he might have been running.
"There you are," he said. He couldn't say anymore however, after that, because Remus had crossed the room and was nudging him back against the desk with Sirius's letter clutched in his hand between them, and pressing his mouth against his. Sirius brought a hand up and threaded his fingers through Remus's hair, completely involuntarily though once they were there and he realised how soft Remus's hair was, he had no problem leaving them there.
He was kissed, softly but thoroughly, until Remus stepped back and looked at him very sternly.
"That is what you should do," he said. "Honestly, do I have to tell you everything?"
Sirius smiled, used the hand still buried in Remus's hair to bring him back for another kiss, and thought about how Remus didn't know him as well as James did, but he knew enough to come find Sirius, and kiss Sirius like that, and-
"Stop thinking, Sirius."
-that was why Sirius fell in love with Remus, instead of James.
