In Equestria, the only season ponies can experience until they meet their soul mate is winter. When Sunset crossed over to the human world, she met her soul mate, and was able to experience the other seasons. The problem is, she isn't in Equestria anymore, and her soul mate is human.

And Sunset doesn't want to force that onto Rarity when to humans, the term soul mate applies to something that isn't real.

Cover art designed by the very generous Novel-Idea! Thank you so much!