Being born within the Moon, with nothing but a The Big Sun who basically raised him for billions of years. A tight bond is bound to form. Even when they've been apart for a thousand years.

So the creator clearly needs some entertainment while working on her current long/multiple chaptered book. So Celestia and SolarFlare from MLE are the victims of this little thing going on. While everyone else sits back and enjoy the show. But what started off as a fun game.. turned dark.

The last words the Moon said to her Sun, were the most unforgivable words that no one should here.. and The Moon had aid it before she and The Sun had officaly parted way.

The Sun has many sides that we are unaware of, Celestia hopes to keep it that way.