There is not good reason why people hate Justin Beiber but in my opinion you will have people who don't appreciate what you do when you are successful. Some people have criticized his music style while other people pick on him because of his love life.
When I was a teen gal I liked manly dudes. I think a lot of chicks were that way, and guys tried to be manly. Now all these young girls are head over heels for the least tough, manly boy ever and dudes are like "What the hell?!". Young girls are making young boys feel like they have to be wimpy douches to get girls to like them now.
People don't like others who are more successful than them. It's just a basic human emotion, to envy something/someone. When someone is all over the place, like Justin Bieber is, they get tired of hearing and seeing their success. Envy and jealousy go hand in hand, Justin could do or have anything he wanted, all the people who say he sucks and hate him, can't do those things. Just like some people can't stand lady gaga, it's your taste and your opinion. So just think what you want to think. I honestly think the kid has talent.