What if you had the opportunity to go to Equestria and realized, too late, that it wasn't what you dreamed of? That it was nothing more than an elaborate illusion.
For years, Sunset Shimmer has held her place at Celestia's side. She was her prized student, her number one prodigy. In return, Celestia was Sunset's world, her passion. However, with time and age come change, and Sunset cannot take it any more.
It seems that no amount of stalling, negotiation, or bargaining will stop the council. Since Twilight's coronation, they have insisted that she be tied to a Canterlot noble, so as not to lose their hold on Twilight's dominion. Tied in marriage.
For quite some time Night Light tolerated his wife's trysts with the princesses but when she finally runs off with thm, he decides that it is finally time to file for divorce and make her pay for the anguish she has caused him.
The ponies are use to living with what are basically gods: The Alicorns of the Sun and Moon, a newly ascended unicorn, and even a chimeric lord of chaos. The Tree of Harmony suddenly having a mobile form? That will take some time, on both sides.