>>86670
In the most general case, it would be because she had asked me to spank her.
She'd enjoy the experience of being spanked, at least in hindsight if not while it was happening to her, and would agree to spanking being used for discipline/entertainment purposes. She'd have a safeword to use if things got out of hand, of course, with the possible exception of more serious punishments.
More specifically, she might get spanked for not doing her homework, failing a test, staying up past her bedtime, wetting her pantsu/bed, answering back, or simply for looking so cute that I couldn't resist. There'd be other punishments and rules too, some of which could be very humiliating for her.