Princess Luna was saddened that ponies did not show appreciation for her beautiful night sky one thousand years ago. Her sadness turned into bitterness, and her bitterness turned her into the terrible Nightmare Moon.
One thousand years later, Princess Luna was given a second chance to rule the kingdom of Equestria by her sister's side once more. But still that same saddening thought lingered in her head.
So the Mare of the Night takes flight to the far cold mountains of the North, and there, she sings her woes, and ponders over the answer to the question that cost her everything she had all those years ago.
Inspired by a scene from the 2013 animated Disney film, 'Frozen'.