After the events of https://derpibooru.org/2402894 Rarity felt under pressure to have herself shrunk in turn, but only Twilight, Rainbow and Pinkie had yet to take a turn with a friend-turned-slave. Rarity’s initial intention was to attend to Twilight but Pinkie Pie busted in during the enchantment, causing Twilight to mis-align the nitty-gritty. The result was that Rarity felt more drawn to Pinkie, but the compelling aspect of the spell wasn’t strong enough and, as such, Pinkie started to get bothered with Rarity’s evident reluctance to do anything that would get messy.
 
While Rarity was meant to be enthralled to Pinkie Pie, Twilight re-examined the structure of the spell and wondered if perhaps they should re-work a few aspects so that each participant’s personal traits could be translated into a role they could perform for a master who they’d be happy to serve even before they’d be subjected to this spell.