I dunno what it is, as a kid I used to quite like them, even cheered them in the two European Cup finals.
I don't think I really started to detest them until 09/10 when they had a poor season under Rafa Benitez and all of a sudden it was like the end of the world. They seemed to be in the media every day whining on in that bloody horrendous accent.
I don't think I've quite despised an opposition player more than Suarez for a long time though, even before all this shit with Evra. Probably going back to the World Cup where he cheated Ghana out of a semi-final place and couldn't hide his glee. Cheating little wanker.