There are a few questions raised by the TV show that you may have picked up on. I've gathered my findings and theories here in order to spare everyone a great deal of unpleasantness. Pass this note along to other humans after reading it. I hope you'll have the good sense to never bring up these matters in public conversation. They are very personal, very painful subjects for all involved.
Voice them carelessly or disrespectfully at your own peril.
Celestia and Luna being 'Princesses' rather then 'Queens.'
When Equestria was first founded, many desired to have positions of power in the new order. The simplest way to accomplish this was to marry one of the princesses. Some historians paint a picture of the last conflict between the three Pony races, a bloody shadow-war being waged to promote their best as suitors while eliminating any rivals. Some modern plays and literature touch on the subject, but always cast the events in a more comedic light. Regardless, both Celestia and Luna were eventually wed.
But whatever the true character of their suitors, any dreams of dynasty and one race being first-among-Ponies were quickly destroyed. With the Princesses' longevity, control of the throne would never pass to their children. In time, anyone who may have harbored such schemes, along with those offering genuine love, simply grew old and faded away.
After the death of their husbands, Queens Celestia and Luna were technically Princesses once again, a title they have retained ever since.
As painful as the prospect of outliving their spouses must have been, the Princesses would each fall in love a few times more, and the cycle would repeat. Unable to take the throne of an immortal ruler, the descendants of the Alicorns turned the wealth and influence they were born into towards forming a new caste in Equestria's social structure – the nobility. The question of whether they contribute meaningfully to the country or merely leech off its wealth depends entirely on who you ask, and on who might be listening. They are still the descendants of the Princesses, after all.
The emergence and sealing of Nightmare Moon
About 1100 years ago, Equestria was still an untamed land. Dangerous creatures roamed deep into the country on occasion, hunting Ponies indiscriminately until being driven off. The night was thus a dangerous time, when the worst of monsters would be boldest and all decent Ponies were barricaded inside their homes. Never celebrated as loudly as her sister, Luna was soon viewed more as an entity to be placated then revered, even though she had no influence over the monsters that roamed the night,
A person uses magic according to their nature. As resentment built within Luna's heart, it was reflected in her appearance and powers. Ponies viewed her transformed state as the true face of the creature of evil some had always thought her to be, and the cycle fed upon itself. At some point for reasons unknown, this process accelerated dramatically until Luna refused to lower the moon one morning, and she was revealed to have completely changed in personality.
With no warning and no alternative, it was all Princess Celestia could do to seal Luna into the moon. Intrinsically tied to the celestial body, it was hoped that re-connecting Luna to the source of her powers would help Celestia to purge whatever had corrupted her sister. However, Celestia's connection to the Elements of Harmony was then broken, turning a place of healing into a prison for the being that now called herself Nightmare Moon.
Celestia and Luna losing their connection to the Elements of Harmony
The explanation offered among the scholars willing to speak on the matter suggest that in being forced to fight her sister, Celestia lost something – perhaps she thought was she had done was too cruel, or felt she had failed her sister, or despaired at what she had been forced to do. Banishing her sister had caused a wound that crippled her ability to feel kindness, laughter, or loyalty to the point that she could never attune to the Elements as she once did.
Likewise, the violent shift in Luna's nature could be reversed, but never forgotten. Traces of the Nightmare Moon persona remain in her psyche like a physical scar. What Luna suffered through has so far blocked all attempts to reform her connections to the Elements of Harmony.