Lovers have their reasons. To Twilight Sparkle, the reasons she loves Celestia are perfectly obvious. And to Princess Celestia the reasons why she can't love Twilight back are equally obvious.
Princess Celestia is sometimes an enigma. What is it with her and tea? Why doesn't she drink cider? And how will she react when Twilight Sparkle confesses that she has a crush on her former mentor?
Celestia likes her routine. Tax reforms, grants, laws and construction projects are all a princess needs to be content. It's just her luck that for Pinkie Pie, "content" is not nearly enough.