"So," Q says slowly, "my desire not to die was stronger than his to live? That's why I went into the mirror and he didn't?"
Q shakes his head. "My only personal experience with them was what we did in class." He picks at a fingernail. "I was tempted of course," he admits, "but I didn't want that. It wouldn't have been right." And you would have hated me for doing it, he adds silently.
"Accidents do happen Severus," he says innocently. "That's why they're called accidents."