Rainbow Dash had her loyalty pulled apart. When Celestia and Luna declared war against each other, Rainbow had been seperated from three of her friends, and many more. While out on a mission, she meets a stallion of great power at her enemy's side.
Rainbow Dash has been invited to Pinkie Pie's wedding. But Pinkie requests everypony who attends her wedding bring along a partner of their opposite gender, and whoever catches the flowers must kiss their partner. Rainbow isn't comfortable with it.