As a new leader and higher up in the ranks of the alicorns, Twilight must be tested. She needs to be prepared to deal with the responsibilities of her duties as a princess. Not only in the term of protecting others, but in physical pain that she would potentially go through.
Or that's what Celestia keeps telling herself. Celestia constantly attempts to kill Twilight, bringing her back to life every time she stops breathing. Twilight doesn't understand. Why would her mentor put her through this? Why would she make her suffer like that? She claims it's to teach her how to deal with pain...
But is she really telling the truth, or does she need an outlet for the pain she suffered?
Special thanks to Mr101, Skeeter the Lurker, RainbowBob, and Merc