Twilight has never liked Discord, even after Fluttershy befriended him and brought him ostensibly to the side of good. And she's always happy to visit Fluttershy in her home without a whimsical and almost unstoppable Chaos God to complicate things. That Discord isn't here for Twilight's visit is wonderful news!

At least until Twilight sees how sad Fluttershy is, and asks her why. Discord is on a mission for Celestia, and it's incredibly serious.