Some people think it's cool to hate on celebrities. Maybe they get a kick out of it. Perhaps they are jealous.
Personally, I just listen to music that I like, but I find it infuriating on the video to see comments like "the dislike bar is the size of justin beavers dick".
Just listen to the music you enjoy, and let others listen to what they want.
Personally, if I was Justin Bieber, I'd have all the success, money and fame so I'd just laugh at all the haters.