Why would Twilight be granted Alicorn-hood simply by completing an ancient spell? Because, if you think about it, Star Swirl's spell switches ponies' places in life, e.g., it makes them understand each other, something that would be paramount in a world where friendship is so heavily relevant. It's rather fascinating to think about.Celestia tells Twilight that her knowledge of friendship was what allowed her to complete Star Swirl's spell. Recalling what Twilight was like in the very beginning, it appears that Star Swirl was the type of pony Twilight would have become without the influence of her friends: an accomplished unicorn with massive magical talent, but lacking the understanding that 'friendship is magic' and all that came with it. Perhaps she would have been content with her books and original conviction that she didn't 'need' friends.Pearls of wisdom straight from TV Tropes.The very first song we were introduced to in this season was Twilight singing about that, despite her copious and clear talents, she isn't prepared for the duty ahead of her. And then the very last song in the third season we hear is how everything is going to be fine. It seems practically as though the last episode was basically an answer to the first. No, Twilight was certainly not prepared for the responsibility that was suddenly thrust upon her...but just like in real life, just because you aren't prepared for something that lands in your lap, it doesn't mean you don't have the talents to make it through with flying colors anyway. It's more like a Fridge Heartwarming, a way of saying "just because something huge and unexpected gets put on your shoulders, it doesn't mean you can't make it work"...which could also apply to the show itself! "No, you fans weren't prepared for this big change, but you know we're talented and we can make it happen." Frankly, an awesome message in my opinion!