What reason does Iago give for his hatred of Othello?

The
question of why Iago hatesso much is one that has occupied scholars for centuries, for although
Iago certainly does offer his own reasons in the play, they do not seem to tell the whole story.
From his own mouth, however, Iago's reasons are, he tells Roderigo, "hearted," just as
Roderigo's "cause" isRoderigo being driven by his desire for Desdemona. In the opening
scene of the play, we discover that Iago has professed his hatred for Othello several times to
Roderigo, his supposed confidant; it is questionable, however, as to how far we can believe Iago
tells Roderigo the whole truth, as he later seems to have little regard for Roderigo's
life.

Iago tells Roderigo that his chief issue with Othello, at this point in
time, is that Othello has rejected the suit of "three great ones of the city" who had
urged him to make Iago his lieutenant, instead choosing "a great arithmetician,"
Michael Cassio, who "never set a squadron in the field." Iago believes that he himself
is far worthier an officer than Cassio and is furious that he has been passed over for promotion
in favor of this lesser soldier. This seems a sound enough reason, and indeed Iago uses Cassio
in his scheme to bring down Othello, as if to punish them both. Iago also mentions that he
believes Othello "hath leap'd into my seat," or that "twixt my sheets / He hath
done my office"that is, that Othello has slept with Iago's wife, Emilia. However, there
seems no justification for this suspicion, and even Iago himself says he doesn't know whether it
is true. The idea is simply posited by Iago as if to further justify his
actions.