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.