It's been 2 months after Starlight Glimmer and Twilight Sparkle became allies and friends after they fought between each other in multiple alternate timelines.
Now she lives in Ponyville Castle of Friendship with her new teacher - Twilight and Twilight's dragon - Spike.

During one of her friendship lessons with Twilight, she accidentally took Twilight by surprise and confessed love to her, which made Starlight embarassed. One thing she doesn't know is that Princess of Friendship feels the same towards her.
Moreover - she has perfect oppurtunity to show love to Starlight - all her closest friends (including Spike) are now on the trip to Fillydelphia, which means she will be with Starlight alone in the Castle.
Twilight wants to propose to Starlight a private friendship session with her, but not in its form she is teaching her.