My reason is that, well, I identify strongly with her element. I'm not appearance-conscious like her... in fact, a lot of my surface traits are the opposite of hers. But in her willingness to give things up, I find her extremely relatable. Twilight is probably the pony I'm most like overall, but definitely doesn't have the same "core" as Rarity does.
I suppose seeing yourself in one of the Mane 6 is another reason.