A treatise on the Elements of Harmony, because god knows how long it'll take me to finish Project Harmony, if ever.
The elements of harmony had their origin in the Alicorn Sisters, Celestia and Luna. They initially manifested at the point when Luna had received her Cutie Mark, marking the power of the two sisters having reached maturation and the strong bond between them. Specifically, the elements manifested as the tiaras and necklaces that Celestia and Luna wear. Initially, they were only an outward manifestation of the fact that their powers and harmonious link had fully-grown, and had no inherent power of their own. The elements were thus initially intrinsic to the sisters themselves, and were thus used to defeat Discord initially. In essence, the sisters were the original elements of harmony: two ponies to represent the fact that harmony cannot exist in a vacuum.
The elements changed when Luna was corrupted into Nightmare Moon. Her corruption broke her link to the elements, permanently (more on that later). The corruption also made Nightmare Moon significantly stronger than Princess Celestia, for she was a new agent of disharmony (possibly linked to a higher force behind Discord?). Celestia needed the power of the Elements to be able to hope to overcome Nightmare Moon, so she utilized a powerful Memory Spell to simulate the bond she had once shared with her sister, to achieve that state of harmony and be able to strike against Nightmare Moon. This, however, was against the fundamental nature of harmony, which cannot exist in a vacuum. It gave her the power only to banish Nightmare Moon, and not to destroy her, because she was attempting to embody all of Harmony personally. In attempting to assume her sister's power, she was later forced to assume her sister's role, and so it was her borrowed power over the Moon that allowed her to banish Nightmare Moon. (side note: the "stars will aid in her escape" thing is a counterspell Nightmare Moon wrought at the last minute, trying to summon the power of the stars since Celestia had hijacked the moon, but she was unable to complete the spell, only to barely put it in motion, taking a millenium to complete).
In unifying the power of the elements, Celestia too eventually lost her link to them, for in gathering their power to her, she had violated the nature of the elements, and so the harmonious power rejected her as well, fracturing into six distinct elements, which had been distinct components of the bond she had shared with her sister. Celestia and Luna's relationship had represented Yin and Yang, night that follows day, and how each element of harmony (aside from Magic) has a "give" and "take" to it that each sister embodied.
Honesty: One must tell the truth, but must tell it to another (Luna was the honest one, Celestia, in her more diplomatic nature, received Luna's honesty)
Kindness: One must be kind, and another must receive it (Celestia was the kind one, bearing Luna's indiscretions with grace and attempting to placate her sister)
Laughter: One to spread laughter, another to laugh (Trollestia the prankster, while Luna craves amusement)
Loyalty: One to be loyal, and one to inspire loyalty (Celestia the elder sister inspired, Luna followed. This is the first element that was broken in Luna's corruption)
Generosity: Gifts must be given somewhere (both sisters were equal on this)
Magic simply signified the magic power behind all of this, that gave it its true force.
So the elements schismed away from Celestia, manifesting themselves in their six discrete components, that would later be uncovered by the chosen ones who embody each element in its entirety (both its Yin and Yang components)
Hooray for wasted time...
Monster Hunter: pissing me off since 2010.
