Celestia reunites with her former student over tea after a forty-year vacation spanning the world, only to find that Twilight has changed quite a bit more than she expected.
"When I read their letters... I think about how temporary everything really is." A lonely princess pores over her studies in a frigid castle, desiring nothing more than the warmth of an old friend.
I have heard ponies say that living so long would be a curse. That it is a burden. That it can break a pony's mind. There is some truth to that, but it's not the whole truth.