In my opinion most haters seem to be people that either have a knee jerk reaction to fans of someone. Example, Artist A has a pretty vocal group of fans that praises their material. The hater then is activated and instantly calls everything Artist A does overrated.
or
The hater was a fan of Artist B at one point. It no longer became cool to like Artist B, they stopped charting, being popular etc. and the former fan became a hater to save face.
or they like an artist and don't want to admit it.
