Trixie doesn't understand how or why, but Twilight Sparkle has become her student with the ambition to learn stage magic. Why is an alicorn of such great magical ability having such a hard time with illusions? It's nothing Trixie can't handle. Trixie is Great and Powerful after all. Trixie thinks she has this in the bag.
To encourage her new student, Trixie offers a special reward. One no sane pony would ever offer The Princess of Friendship. She was only trying to be nice. Poor Trixie.