Rainbow dash was bullied because of her unintentional mistakes she made, she had enough of it and quit then decided to join the Equestrian Air Force to make a living and to find better comrades. She joined and even though it was rough, she made it in.

Note: I don't own mlp or the cover art they belong to their rightful owners. the cover art is from someone else i just drew a hat on Rainbow dash to fit the story.