When Twilight became a professor at Celestia’s academy, she worked beside Professor Sunset Shimmer. The pair were linked in that their subjects were very niche. Twilight taught ancient magic theory while Sunset debated dimensional planes. Being butted out by the more conventional subjects, the pair found a friend in each other.
It wasn’t unknown that Twilight carried a torch for Sunset, she did lay it on quite thick. Sunset reciprocated these feelings quite strongly. The pair lightly flirted in public and privately,,,, well, they were much more upfront privately. The pair were happy.
However when Sunset’s theory of other dimensions was revealed to be true and she found a way to traverse these worlds, the mare was threatened by Celestia to cease. The two debated, these debates turning into blazing arguments. Twilight had to make a choice, whether to follow her love or defend her Queen.
Twilight chose to defend and so Sunset disappeared across another dimension, effectively ending the two’s relationship. In pain, Twilight had multiple, unserious, relationships. While these ponies loved Twilight, she just couldn’t bring herself to jump that gap and so left a lot of heartbreaks in her wake.
Then Twilight meets Applejack. She can’t understand it but this was different for her. She didn’t want to ruin this like she’d done to all the others, so she tries to keep it to herself.
So life goes on, Sunset appears once more to brung destruction to Equestria. To prove her power… she got wrecked. Twilight didn’t want to hurt Sunset, so was more than happy to reconnect even before Sunset’s prison sentence and probation period. While there was still a lot of pain and betrayal between the two, they couldn’t deny they had missed each other. They still cared for each other.
That was very evident to Applejack. Why should Applejack care? Sunset was a friend now. Who cares about their history?
But when Twilight laughs a little too loudly at Sunset’s joke…
When Sunset gets a little too close to Twilight…
When the pair would linger over each other, faces flushed….
Applejack could feel that streak of jealousy rising up her back.