Because sometimes, she loves math so much that she just wants to do a whole week's worth of math lessons in one day. And since I'm the teacher, that's just fine with me.
And sometimes, we've worked on a concept for weeks without it really making sense to my young pupil. And then suddenly, putting numbers in the tens or the ones place just clicks, and the purpose of the number line is no longer so mysterious, and double-digit addition is no harder than single-digit addition.
And I get to watch the excitement of mastering a new skill unfold right before my very eyes.