YUI just... gets on my nerves so much. :l
For one, she always has this look on her face like "I'D RATHER DIE THAN BE TAKING THIS PICTURE ARWRAWRAR"
And her music bores me to death; every song I've heard by her was dull and they all sounded the same to me. And I really dislike her voice.
Plus, I had a friend who was all like "Ayu SUX YUI is so amazing" and it really fueled my dislike for her.