I don't "hate" the guy. But I dislike his attitude and I root against him. He is the best player on the planet, hands down. But as for why I dislike him, I don't know. How about . . .
1) the decision
2) his (and Bosh/Wade's) arrogance in asserting they would win at least six or seven titles
3) his bad attitude and sense of entitlement in dealing with the Cavs organization and coaches (Silas, Brown)
4) his grandstanding (e.g. the block on Tiago)
5) his whining to the officials
6) related to #5, his lack of effort when he doesn't get a call he wanted (happens dozens of times a game).