Solar Flare

It is an entirely unique experience to be a student of Princess Celestia. Therefore, Twilight Sparkle and Sunset Shimmer understand each other in a way which no other pony can. That plus so much else they have in common made this inevitable. As for Princess Celestia, for her this must either be a pleasant dream, or an AU for sure. Princess Celestia takes students for very real and very good reasons. Training her eventual successor is a serious matter, and the realm has need of highly qualified ponies all across the lands. But it was never competition and contention that Celestia desired to create. So this blossoming love, which nothing can ever ruin, is the manifest essence of all the blessings Princess Celestia has always sought to bestow upon Equestria.