Personal theory time: Celestia has just spent a ~millennium alone (I'm assuming Cadance aged normally here). That's got to be a little bit traumatizingly depressing. Then, in the span of a less than two decades (again, assumptions about pony lifespans being made) Cadence comes along, she gets her sister back and her student attains princess-hood. Scroll over this sentence for my spoiler based thoughts. Edit: I was right! It was about pride in Twilight.