In the Season 5 premiere, fans are introduced to Starlight Glimmer. We see her phony philosophy about equality, misguided views of friendship, and her strange hatred of cutie marks. Yet we know nothing else about Starlight's character. Where did these obsessions come from? what cause her to act this way? How does she know such powerful spells?
She already made an easter eggs appearance in two Twilight-centered episodes. So for while I've been thinking; what if the issues in the episodes of season 5 are hinting the fans about who Starlight really is that will be revealed in the season finale? Bloom & Gloom, Appleoosa's Most Wanted, and Crusaders of the Lost Mark are examples of disliking a cutie mark, which may explains Starlight's hatred for them.
Tanks for the Memories — Realizing life is not fair and dealing with "lost".
Princess Spike — The feeling of not being the princess.
Do Princesses Dream of Magic Sheep? — The need to hate oneself.
Canterlot Boutique — Having things done the way you like it and not how someone else wants it.
Rarity Investigates! — Putting pride ahead of decency.
Hearthbreakers — Issues with family.
Even some sources outside might shed some light.
After seeing this trading card and how it might be a Season 5 spoiler, it's possible it has something to do with Starlight.
My Theory: Starlight Glimmer is actually the descendent of Star Swirl the Bearded. Her family has been mastering his advance spells for generations. They have been trying to solve Star Swirl's unfinished spell with no success. At the time Starlight was meant to inherit the spell book, Princess Celestia have already sent it out to Twilight and she solved it. She became an alicorn princess out of it all. Starlight became furious that her family's legacy was ruined because of some other pony who never intended to be a princess. It was long until she found out that the Tree of Harmony, which bared Twilight's cutie mark, meant that she was destined to be a new princess. Then the tree gave Twilight her castle and the role to spread friendship throughout Equestria. Starlight felt insulted. Everything that Twilight was given was meant to be hers. Star Swirl have started that unfinished spell, and Twilight came along and finished it. Now she is the "Princess of Friendship"; all because of what a princess told her, what a tree told her, what a cutie mark told her. So Starlight defied destiny, defied princesses, and defied cutie marks. Her bitterness drove her to spread the magic of friendship 'her own way'.