Ok I kind of got this idea when I saw a humanized Nightmare Moon Cosplay so just hear me out.

What if the reason why Luna turned into Nightmare Moon was because she was left at the altar?

(I know her transformation is now cannon but shut up)

I 'unno. Try it. =3

I actually think something like that would anger her enough that shed want eternal night and evil stuff. Damn... Hell hath no fury like a mare scorned