Why does lago hate Othello?

Iago hates Othello for a number of reasons. He's an old soldier, a military man to his fingertips, yet he's been passed over for promotion by Othello in favor of Michael Cassio, who only gets to be promoted, thinks Iago, because he's an aristocrat. Iago is of a much lower social class than Cassio, so it's not surprising if he has a bit of a chip on his shoulder for what he perceives as a serious snub.
Iago also hates Othello because he believes that he's been having an affair with his wife, Emilia. There's no evidence of this, just rumor, but that's enough to make Iago want to carry out a terrible vengeance upon the man to whom he's supposed to be loyal. Related to this point is Iago's racism; he hates Othello because he's black. And in common with many racists, he regards black men as being sexual predators, always looking to prey upon white women.