How you know all of them (Pinky, Dash, Rarity, Twilight Apple Jack and Fluttershy) are friends. But After many years, Twilight has been given a book which is probably 50 yeas old. But two lines and one chapter change each one of them. There's a secret in those two lines which Twilight should think of. Dash and Twilight are more than friends, friendship between them, is not what you think it is :)