>“He did not understand friendship like you do”.
>Discord says that Fluttershy is her only friend.

I think we have another argument to that fan theory about Discord and Starswirl.

So… Starswirl, a very powerful Unicorn mage, lacked the one ingredient that he needed to complete his spell: friends.
Now, we can all safely assume that Starswirl's Talent was related to magic and spell creation, possibly even spell creation itself (considering what Twilight once said about how he created a shitload of spells). We also know what happens when a character fails to live up to their special Talent: insanity.

So, Starswirl goes mad because he cannot finish the final portions of a new spell, and ends up becoming Discord. Then, he wreaks havoc upon the world of Equestria with his unrestrained power.

FUND IT.