I cackled at the thought of a pixie — perhaps one of Lockheart's — alighting on Hermione's face.

The wicked humor within that brief moment of banter could become a lynchpin in their eventual relationship. It (slightly) humanizes Severus from inscrutable Professor to a man she'll come to know. It also gives her a chance to start their bonding off as something close to his equal. Naturally, hardly anybody is going to see them as such for a while, but it's an auspicious start. =D