Fluttershy and Rainbow have been friends since they were kids. Always together, never too far apart. It's a scary thought that it probably won't be that way forever.
It's the hours after the disaster that was the Gala. Rarity tries to pretend her feelings weren't crushed by all of it and fails miserably. This gets noticed.
Rarity always wanted a dream wedding. Now that she's about to marry the dream girl, she's determined to make the rest just as perfect. Of course she'll overthink this.