weeeellll, i personally love rarity and dislike fluttershy, but i wouldnt go as far as to say every loves rarity and everybody hates fluttershy. i personally havent even noticed much love coming to rarity. as a matter of fact, ive spoke to people who absolutley HATE rarity. and ive spoke to people who hate Fluttershy, or Rainbow Dash or Twilight or Pinkie or AJ or any other character because the fact of the matter is that every brony has a different opinion on different characters, you feel me? Now, i will answer the question you orginally asked but im going to answer it slightly altered;
"why do YOU love rarity?"
now, I love her because her element is very close to my heart, i value generousity very much and try my hardest to be a generous friend. that's how i fell in love with rarity. but once i started liking her, i realized i started liking other things about her such as her voice, her design, and mostly her personality. I loved how prissy she was! but she still managed to make time and help out her friends. that's why i love rarity, but everyone has their reasons, as i said. anyway, my point was; don't over-generalize the entire fandom by saying "well, everyone likes *insert name here* and EVERYBODY hates *insert new name here*. See what im saying? I personally think its great that rarity is getting some love *w*