I chalk it down to autism, ironically a lot of people who say this can’t pick up on what a nuanced personality really is. Fluttershy is the most one dimensional of the two because her personality is just quiet compassion and social anxiety. That’s appealing to people but it’s also incredibly one note, and it doesn’t make her interesting if she occasionally gets angry or whatever. Rarity on the other hand has flaws that she acknowledges and tries to overcome. Seeing her be jealous of fluttershy and then admitting that to herself and wanting her success anyway is a feeling everyone goes through at some point and is a big part of growing up and maturing as a person. Fluttershy never matures or deals with herself emotionally in a reflective way, she’s just a sort of stock stereotype cute character who you are meant to gush over for being adorable. But rarity is the one you can relate to more as a personality, she’s a hard working girl and she can recognise her flaws and overcome them to do good by her friends. Rarity teaches valuable lessons about how to behave like a responsible adult