He's a werewolf! That's why he's been missing classes!

“Why were you lurking under our window?""Yes - yes, good point, Petunia! What we…”

“I never need to find time to read. When people say to me, ‘Oh, yeah, I love rea…”

“A letter?” repeated Professor McGonagall faintly, sitting back down on the wall…”

“It was as if Snape had started handing out sweets.”