For Twilight, the increased lifespan that came with princesshood had forced her to outlive her friends, but thanks to Rarity, she will learn that centuries could be lived in but a single day.
The princesses gave their magic to Twilight to prevent Tirek from stealing it. As a result, they end up stuck in Tartarus. The fact that things manage to go downhill here says a lot about the wonders of family.