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.