A theory I've always had about the Elements of Harmony is that apparently, each one individually possesses a strong level of magic in of itself. Also, each element will release a certain amount of its magic, depending on the wielder/possessor of that element, and how much of that element they represent.



Confused? You more than likely are.



When Princess Celestia defeated Nightmare Moon with the Elements of Harmony, she wielded all six at once. Now, combined, their magic was far too powerful for a single alicorn princess to wield. This is sort of the basis of my fanfiction: She realizes they were too powerful for a single pony to use. Even when she and Princess Luna used them together to defeat Discord, Princess Celestia knew it wasn't enough; there was just something about the Elements that made their magic far too powerful for a single alicorn, or even two alicorns, to wield.



I think the Elements are only as powerful as the pony who wields them. In truth, any pony, especially unicorns, could wield the Elements, either individually or combined, but it's possible it depends on how much of that Element the pony wielding it possesses or represents in their heart. How much of their personality traits or characteristics fully represent that Element?



You know how some RPG games, like Dungeons and Dragons, have different levels of statistics for things like armor, weapons, magic, health, and other things? Think of every single pony in Equestria as being a character in a RPG, and the Elements of Harmony being their statistics. Each pony would possess a certain amount of Laughter, Honesty, Loyalty, Kindness, Generosity, and Magic. Some ponies might possess just a little more of one particular Element than other ponies, but no pony could possibly possess a very high level of one individual Element.



I know a Unicorn (Twilight Sparkle) activates the magic the Elements contain, but even a Pegasus or an Earth Pony could quite easily activate them. They both have their own style of magic, although it seems that only Unicorn magic can fully activate the Elements.



Going back to what I said about the levels of the Elements. Only a pony who truly possesses the characteristics or fully possesses the Element, could, on their own, unleash the full potential of the Element. Any single pony could wield the Element of Kindness, but only a truly kind pony, one whose heart is full of kindness, such as Fluttershy, would be able to unleash its full potential. Any pony could be loyal to their friends, family, or home. Any single pony would be willing to fight or protect those they love, but only a truly loyal pony, who would stick by their friend's or family's side in any sort of situation, and even give up on their dreams if it meant leaving their friends or family behind, such as Rainbow Dash, would be able to possess and unlock the Element of Loyalty.



In short, the Element is only as powerful as the pony who wields it. No single pony is capable of wielding all six all at once, unless they somehow managed to possess strong, equal amounts of all six Elements. This is why Princess Celestia failed. Had she been able to use all six Elements efficiently against Nightmare Moon, she would have defeated the entity that possessed Princess Luna, and Luna would have been walking/flying around with Princess Celestia. There would have been no Mare In The Moon. Discord would have been fully destroyed, not just simply turned to stone, if both Luna and Celestia possessed equally strong levels of all six Elements. I think Princess Celestia imprisoned Luna on the moon, for safe-keeping, until she could figure out more about the Elements. She stored the Elements in their casings on that stone dais in the old castle until she could figure out what to do.



Enter the Mane 6. I think she saw potential in all six ponies, especially Twilight. When Twilight's magic showed itself after Rainbow Dash's Sonic Rainboom, Princess Celestia knew she'd found the Element of Magic. She also knew this was the strongest and rarest Element to wield, so she took Twilight on to be her student for private lessons. Once she felt Twilight was ready, she sent her to Ponyville to check up on preparations for the Summer Sun Celebration, and instructed her to make new friends. I think Celestia found the other five Elements in Ponyville, which was why she felt it was time for Twilight to visit there, and probably picked Ponyville personally to hold the event. And it was no sheer coincidence that the rest of what would become the Mane 6 were chosen to take lead or have the most important jobs in preparing for the event: Rarity decorating Town Hall, Fluttershy training a flock of birds to sing a fanfare (this is probably the most interesting, having her train a flock of birds versus using royal trumpeters), Rainbow Dash clearing the skies, Pinkie Pie organizing a party for everypony to stay up the night before, and Applejack being in charge of the food (Why her? Why not the Cakes, who would have been more well-prepared?). Every single one of these ponies have always possessed traits of the Element they are now associated with. These ponies each possess that one Element more than any other pony in all of Equestria, and it is because of this that the Elements of Harmony are now stronger than ever.