“I don’t know the first thing about friendship.” - Sunset Shimmer
Or maybe, just maybe - you’ve forgotten how to be a good friend, a good pony and a good person.
It takes rediscovering an old friend to remember just how much you’ve changed.
Headacanon: Twilight Sparkle was given a dragon egg - Sunset Shimmer was given a phoenix egg. When she managed to conjure a ‘spark of life’, giving life to the egg and causing a phoenix chick to hatch out of it, Sunset was given the bird to care for. The phoenix became her study companion, her assistant, her helper and, mainly; her best friend.
Celestia kept Philomena as hers as soon as Sunset Shimmer crossed the mirror, both of them persistent in waiting for the young unicorn’s return.