Iago's anger is motivated by his injured pride, his ambition, and his sense of outrage superiority with regard not only to Cassio (who has been advanced over him), but also with regard to Othello himself.
Iago's contempt for Cassio is fully explained in this scene. He calls Cassio a man who "of a battle knows more than a spinster," a man of "bookish theoric" or theory who is "without practice, in all his soldiership (795)." This man, whom Iago calls a "counter-caster," has been given preference over Iago who is nothing more than "his moor-ship's ancient (795-796)." This is a secondary position and one of less significance. The "moor-ship" is a caustic reference to Othello, the "Moor," who is not a "lordship" or noble, but who is in a position superior to that of Iago.
Though Iago says that "there's no remedy; 'tis the curse of service, preferment goes by letter and affection and not by old gradation," it is clear that he is not willing to accept having been passed over (796). The resentment of the career officer, intent on advancement in the service, is readily apparent. In addition, Iago goes on to tell Roderigo that he will follow Othello despite his disappointment. His remark is important: "We cannot all be masters, nor all masters cannot be truly followed (796)." What this means is that Iago will simply appear to be a loyal follower of Othello, but he will not actually give his leader the loyalty or respect that is requi