Fluttershy is many things: Animal caretaker, Element of Kindness, and even best friends with a Wonderbolt. But even with everything that she had done and all the things she had achieved, there was still something on her mind.
Maybe it had been all of the times that she had seen her shows. Maybe it was because of how good a flier she was. Maybe it was because of how brave she was willing to be in front of audiences of potential thousands. But whatever it was, Fluttershy had one thing on her mind. She couldn't bear the thought of Rainbow Dash flying alone with the Wonderbolts.
So, whether she was crazy or just really, really determined, she was going to join the Wonderbolts.