We see Twilight and her friends as a happy group of friends from every part of the spectrum, but we also know that they all have dreams. Rainbow and the Wonderbolts, Applejack and her farm, Rarity and Canterlot -- they all have something that they're aiming for, but what would happen if they became who they've always wanted to be? There wouldn't be a lot of time left for their friends, especially if each of them were cities away from the others.
Saddened by their departures, both transpired and imminent, Twilight makes a difficult decision that even she may not understand, and she lets each of her friend's know about what she plans to do.