It all started because Remus asked Severus for help in Potions. A class that Severus, obviously, excels in, while Remus flounders like a fish out of water and tries not to fail miserably. (It would be the only class he fails this year, and how would he explain that to his mother?)
One tutoring session turns to two, then four, then ten, until it becomes this unexplained thing neither of them question or put a name to. It's just Remus—sneaking out so James, Sirius, and Peter don't throw a fit when they find out where he's going—meeting Severus, and making use of the Potions classroom after dinner while Remus tries to learn how to properly measure ingredients and Severus tries to learn patience. Neither of them are very good at it.
Which means Severus finds no humor in a potion-gone-wrong that results in a small explosion which fills the classroom with thick, pink smoke and leaves them coughing hysterically.
"You, Lupin," Severus manages between wheezes, "are an absolutely miserable, useless—"
"I'm terribly sorry! What was it, two scoops of—oh. Two pinches. Yes, that would do it..." Remus waves his hands in front of his face. As the smoke settles, it leaves behind a thin, cotton-candy colored dust covering everything—including him and Severus. He presses a hand to his mouth and desperately tries not to smile. Severus glowers at him, but it's hard to be intimidated with the glare hidden beneath all that pink.
"Your incompetence is so astounding I'm having a hard time believing you didn't do that on purpose. You aren't an idiot, so why is this so difficult for you?"
Remus bites his lip. "I'm not an idiot? Why, dear Snape, I believe you just complimented me."
Severus blinks once, startled, then whirls away. "Clean up this mess. We're done for the night." He stomps out and leaves Remus in the pink classroom in his pink robes with his cheeks burning equally pink. Somewhere between disappointed he's undoubtedly going to fail Potions no matter how many lessons Severus gives him—
—and relieved Severus hasn't realized the reason he's failing is because Remus has been too busy watching him.