Princess Luna, also remembered as the feared Nightmare Moon, was not always so stiff. In fact, after she the defeat of Discord all those years ago, she had started a colony of her own in a haven that she called the Garden of Shadows. Shortly after establishing this colony, she found a lost alicorn babe that had a coat as white as the moon and eyes that were turquoise, much like her own. She took him in as her own son and raised him among the other children of the night, right up until she lost control over her dark powers which led her to become Nightmare Moon. Now a thousand years later, she has returned, and many things have happened, from Discord's own return and becoming good, to the changlings turning into the metamorphesed. No longer is Nightmare Moon plaguing her mind, but something else does, and she fears that this is something she can't forgive herself for.
The guilt of abandoning her children when they needed her most.