Because it wasn't about magic at all, and I wanted to show that the fundamental rift between Lily and Severus was as typical and sad and familiar as any adolescent cock-up. Because that's the age at which most of us engage in side-taking and loyalty-trading, in bolstering our self-worth by demoting unpopular friendships or spurning overtures from kids who don't quite make the grade. It's humiliating and cringe-worthy, and it happens all the time.
Given the meeting at school I just came from, regarding my adolescent and his totally inexcusable behavior (inexcusable except that he is an adolescent, and they do stupid, stupid things but MAN could I ever have done without this meeting), it's both, yes, cringe-worthy and humiliating, but also, it's at least somewhat comforting that it's also common. Heh.
*wanders off to apply consequences of stupid acts* *sigh*