Light and Shadow are not meant to be together.
People in the Light Realm wish to not encounter the ones in the Shadow Realm.
And the people in the Shadow Realm wish to not encounter the ones in the Light Realm.
However, with the war going on, the two sides just have to "get even" with the other.
The prince of the Shadow Realm meets the prince of the Light Realm.
And they realize that Light and Shadow have to be with the other.
They are meant to be beside the other.
*I do not own Fairy Tail or the picture*