It could be for a multitude of reason's:-
1) Height - to tall/short.
2) Weight - to heavy/light
3) Hair - bald/blond/ginger/curly/dandruff/greasy etc
4) Clothes - tragically hip or just tragic
5) Demeanour - to happy/sad
6) Bossy
7) Shy

Don't get me wrong i know lots of short, fat, balding, ginger, unfashionable, sad, bossy and shy people, who i actually adore - but sometimes certain people just really bug me for no apparent reason so one of the above just becomes my apex of hate - examples:-
"I hate him he's tall"
"Look at her she's always happy - she makes me sick"
"He looks like he could with a good dinner the skinny so and so"
"She really get's on my nerves because she is so shy and introverted".

Well why is it?
Am I just hate filled??