in high school, sirius black and remus lupin are rivals and enemies. when they meet again seven years later, they start to notice things about each other they never stopped to think about before. basically rivals turned lovers.
When Remus Lupin starts dating Fabian Prewett in sixth year, Sirius Black doesn't understand why he can't be around Remus anymore. After questioning his sexuality and realizing he's been in love with Remus for years, Sirius strategically avoids his friend until one night when Mary suggests they play spin the bottle with the seventh years.