In Act I, scene 1 of Othello, why does Iago say he hates Othello?
In Act I scene 1, Iago reveals to the discontented Roderigo that he hates Othello because, although he had three people present his "suit" to be promoted to become lieutenant, Othello ignored their words, and instead chose Cassio to be his lieutenant, a man who is younger than Iago and has not seen military service. Iago thinks that he has much more experience of real battle and therefore should have received the promotion. The way that Othello has overlooked him in this way and promoted someone who is "Mere prattle without practice" over him has filled him with rage and anger. Note what Iago himself says to Roderigo about Cassio:
A fellow almost damned in a fair wife
That never set a squadron in the field
Nor the division of a battle knows
More than a spinster--unless the bookish theoric,
Wherein the toged consuls can propose
As masterly as he.
Thus, overtly, as we can see from this scene, Iago hates Othello for the way that he was not selected for promotion, and instead one who was unworthy was selected. Now, of course, Iago must serve under Cassio in the chain of command, which would not help his anger and resentment.
Act 1, scene 1 of this play serves to set the scene and introduce the audience to Iago, whose machinations and deviousness drive the plot of the play. Speaking to Roderigo, Iago declares that he hates Othello because, although he knows he himself is "worth no worse a place" than that of Othello's lieutenant, he has been passed over for Michael Cassio, whom Iago feels is extremely inexperienced and of "the division of a battle knows [no] more than a spinster."
Iago, instead, must settle for being "his Moorship's ancient," an inferior position and one Iago is sure he has surpassed in experience. Iago ascribes this decision of Othello's to the fact that "preferment goes by letter and affection," rather than being based upon "old gradation," or experience.
Later in the play, Iago indicates that his hatred for Othello is also driven by a fear that "the lusty Moor / Hath leap'd into my seat" (that Othello has slept with Iago's wife) but this is not mentioned in the first scene of the play. Here, Iago's concern seems entirely with the issue of having been passed over for promotion unfairly.