Iago is angry that Othello has passed him over for a promotion, preferring instead Michael Cassio, who is not even from Venice, but from Florence. He has chosen Cassio, a "arithmetician" but not an experienced soldier, despite the fact that Iago had the support of three Venetian nobles, and Iago, charging that Othello has not been fair in his decision, asks rhetorically whether he should have to continue to show loyalty:
Tis the curse of service,
Preferment goes by letter and affection,
And not by old gradation, where each second
Stood heir to the first. Now, sir, be judge yourself
Whether I in any just term am affined
To love the Moor.
Iago is complaining to Roderigo, who has been spurned by Desdmona, and the audience discovers that she has eloped with Othello. The two men warn Brabantio that his daughter has married the Moor, with Iago sneering that "even now, now, very now, an old black ram is tupping your white ewe." At this point, the audience sees that Iago hates Othello and will scheme against him, though they cannot begin to fathom how far he will go to get his revenge.
Iago is one of the most compelling villains in Shakespeare. He has two main reasons for being angry at Othello.
The reason that is most commonly known is his jealousy over Cassio's promotion—a promotion Iago expected Othello would grant to him. As we learn later,
“O, beware, my lord, of jealousy;
It is the green-ey'd monster, which doth mock
The meat it feeds on.”
Iago is angry for being overlooked for promotion by Othello:
Despise meIf I do not. Three great ones of the city(In personal suit to make me his lieutenant)Off-capped to him, and by the faith of manI know my price, I am worth no worse a place.But he (as loving his own pride and purposes)Evades them with a bombast circumstanceHorribly stuffed with epithets of war,And in conclusionNonsuits my mediators. For “Certes,” says he,“I have already chose my officer.”And what was he?Forsooth, a great arithmetician,One Michael Cassio, a Florentine....
I am one, sir, that comes to tell you your daughter and the Moor are now making the beast with two backs.
Farewell, for I must leave you.It seems not meet, nor wholesome to my place,To be producted (as, if I stay, I shall)Against the Moor.
“It is thought abroad that ’twixt my sheetsHe has done my office”