There really is no specific answer. Sometimes people just say they hate him to get attention. If they really do hate him, it is most likely for one of these reasons: They don't like his music His voice is not deep He was dating Selena Gomez He is popular
They expect him to be perfect when he isn't. Some dislike his voice, his attitude, the fact that he is popular for what he does. (I mean, what do you expect when about half the population uses him in jokes. He's going to get known some way.) People "hate/dislike" yet they still give him attention.
I can't speak for everyone else, so I'll just give you my answer. I dislike pop music, and his voice. I don't hate him as a person, just his music. But it's nothing against him, I listen to indie music.