The Changeling that Fluttershy watches over was actually considered the caretaker of the group of Changelings. She was like a stepmother who helps her true mother watch over the children. It has a duty to take care of the health and food supply for all of the other Changelings, so it would always be the one who volunteers to find love when the members of the team are unable to find it on their own and that’s why it was chosen to be disguised as Fluttershy, because it had a personality as similar as hers.
Ever since it started to harvest love, it always believed that ponies and other sapient beings simply existed as sources of love for Changelings. It always had to pretend to be so many ponies in order to gain enough love. Not only for itself but for the other Changelings that failed to find love on their own as well. So many masks, so many roles, it trained itself so hard to be the very best actor it could, but in the end, it lost itself…It no longer knew who was it’s true self anymore…
But Fluttershy saved it. She helped it to find its true self again and told it that it didn’t need to be anypony but its true self around her. She also proved to it that sapient creatures such as herself were not the only the source for love. She introduced it to all her animal friends, taught it how to take care of them, and how to make them like us, assuring her that Changelings don’t need to pretend to be anyone but themselves when being around these animals.
The Changeling also discovered that animals do indeed produce love as well, even if it’s not as high an amount as ponies, but it’s still love no less.
Fluttershy also taught it the way of kindness and how to gain love from either people or animals with it. She taught it how to be a better pony, how to be loved without pretending to be or replacing some existing pony. She helped it to learn and create its own original disguise and to gain love from everypony while it acted as its true self. She helped it to become the most kind Changeling in existence.
https://www.youtube.com/watch?v=Gn3caYBZLyc