Sirius lived off dares.
It was James that set him his first, naturally. In return, he dared the teen to ask out Lily. So far, dares had been good.
On April Fools', sixth year, James dared Sirius to kiss Severus Snape: sworn enemy of the Marauder's. Remus had to make sure the dare actually happened (at the time, Sirius didn't have a clue Remus was in love with him, but James did).
Sirius found Severus in the dungeons. He grabbed him and kissed him, then delved deeper and pulled away. He smiled, and then Severus kissed him again. And for some reason he kissed him back.
Remus stepped back a bit, and tried to pretend his heart wasn't breaking.
Severus had many habits.
He'd bitten his nails for as long as memory would allow, he brushed his teeth for precisely 2 minutes 12 seconds every night, he could only ever eat toast that was slathered with beans. Sirius was his newest, and perhaps most favourite habit.
They'd meet in cloakrooms or deserted classrooms and then rush off before anyone could see. Remus would usually hang around outside. Severus always saw him waiting, and one night pointed him out to Sirius.
It was history ever since.
Remus liked systems.
There was a safety in them: they were consistent, and normal. Roughly every month he'd turn into a werewolf; every two weeks his parents would write to him; at weekends he slept in Sirius' bed, Severus usually joining them.
It was hard, sneaking a Slytherin into Gryffindor dorms, but doable. James agreed to help, so long as they let Lily stay during the week; and Peter helped because, well, he was Peter.
Three people didn't really fit into an individual bed, but they managed.
"Ow, Moony, you're on my leg -"
"Sorry, Padfoot."
"Must you call each other those ridiculous names?"
"What? You have one too, Snivellus."
"Fuck off, Black."
"Can you both just be quiet, please?"
Somehow, they managed.