Ponies and Destiny · 3:41pm Feb 19th, 2013
Okay, I want to get something out of the way right now: I loved "Magical Mystery Cure". Everything about it was brilliant, and M. A. Larson and Daniel Ingram are on top form. The animation was the smoothest and cleanest I've ever seen it, the effects were beautiful, and Twilicorn looked completely natural. As such, it might be worth taking what you're about to read with a grain of salt; it's going to be flavored by my own bias.
I've seen a lot of people put off by the fact that Celestia seemed to "force" the alicorn form on Twilight. This was especially jarring to many, as it seemed to contradict the message of the first half of the episode; namely, that you'll never be happy living up to a destiny that you didn't choose for yourself. I, however, think Twilight's transformation fits perfectly with the rest of the episode, and indeed the show itself, for reasons I'll outline shortly.
First, though, I want to clear up a little bit of interpretive dissonance. When Twilight is in that "Celestial Plane", or whatever, it didn't seem to me that Celestia was causing the transformation. She wasn't using any magic (unless she can do it without the aura now), and the swirly magical stuff is Twilight's color, not Celestia's. I posit that Twilight would have transformed with or without Celestia being present. So why was she there? Why, it was to offer a familiar face and a reassuring presence, so that Twilight wasn't scared of what was about to happen.
Now, on to the "destiny" business. Harken back, if you will, to "Cutie Mark Chronicles"; specifically, to Rarity's story. I bring your attention to the lines, "unicorn magic doesn't happen without a reason," and "I knew that this was my destiny!" Destiny is a fundamental feature of Equestrian life. It's intrinsically tied with the cutie mark, and gives each pony a purpose and direction. Each pony's purpose supports and is supported by his/her special talent, which allows the whole system to work as well as it does. The reason the incomplete spell was so bad was that it changed the ponies' destinies to ones they were not suited for.
Becoming an alicorn was Twilight's destiny. That's the simple fact.
Now, all this talk of destiny raises some deep, philosophical questions that I'm certainly neither qualified nor willing to answer. Instead, I leave you with this thought: Equestria is not Earth (though, strictly speaking, Equestria is a country on a planet which, according to "Hearth's Warming Eve", may be called Earth), and ponies are not humans. The rules are different. It's to be expected that some aspects of their universe seem strange to us. Even so, there's no denying that their system works.
I use the Mighty Max version of destiny. Destiny is equal parts need, random chance, and free will. You forge your own destiny, the main limits are actually those you set for yourself. There are some things that 100 times out of a hundred you will always do, no matter what the other options are, and some things you will never do.