One thousand years is a long time to keep a memory alive, especially one so filled with pain and regret, but some things are worth remembering. After all, somepony has to tell her story.
When Princesses Celestia and Luna visit a public art gallery, they come upon a painting of Celestia, banished to the moon. They react to this in the only reasonable way: artistic criticism and analysis.
Sometimes the words ring hollow. Sometimes the ritual feels shallow. Sometimes it's hard to tell if the gifts we are given are from the heart. But sometimes, actions betray our feelings with a power that neither word nor ritual can capture.
There are things a Princess cannot do. Over the centuries, Celestia has been asked to do those things countless times. And the moments when her ponies need her the most are the ones where she can't do anything.
Nightmare Moon is defeated, Ponyville is celebrating the return of the sun and Princess Luna finds herself a thousand years from the world she knew, where even apples are strange.
When Princess Celestia happens upon Twilight Sparkle as the latter fails repeatedly to make a drinkable cup of tea, a new battle will be waged in the ages-old war between the enlightened way of tea and the heretical, dark path of coffee.
Luna has been considering the matter for a while, and she can't put her hoof on why. Confused and irritated, she goes to her sister in search of enlightenment.
For years, Twilight Sparkle has kept track of how little both Princess Celestia and Luna eat. A small slice of cake here. A single cup of tea there. So what really keeps the Princesses going?