Iago’s motivation is clear right from the start of the play. “As early as the sixth line of the first scene of the first act this motive is predicated as basic data for the action which follows. Roderigo says to Iago: “Thou told’st me thou didst hold him in they hate.” Iago answers: “Despise me, if I do not.” (McCloskey 25). Iago is expecting to be promoted to lieutenant by Othello; however, it is Cassio that ends up with the promotion:
“One Michael Cassio, a Florentine,
A fellow almost damned in a fair wife,
That never set a squadron in the field,
Nor the devision of a battle knows,
More than a spinster, unless the bookish theoric,
Wherein the togged consuls can propose
As masterly as he.” (Shakespeare 1.1.20-26).
Iago is upset and feels betrayed by Othello’s choice to promote Cassio instead of him, considering his many years of loyal service. He essentially feels that his military career has been useless since he is being replaced by someone so inexperienced. With this betrayal, Iago has an excuse to hold a grudge against Othello and based on his jealousy, he has also been given a reason to become hostile towards Cassio.
Another reason Iago does not trust Cassio is because he believes that he is having an affair with Desdemona, even though he has never found any evidence to support this idea:
“That Cassio loves her, I do well believe’t;
That she loves him, 'tis apt and of great credit.
The Moor, howbeit that I endure him not,
Is of a constant, loving, noble nature,
And I dare think he'll prove to Desdemona
A most dear husband.” (Shakespeare 2.l.275-280).
Iago’s belief in the affair is based only on his suspicions. The affair between Cassio and Desdemona is part of the plan that Iago has created to destroy Othello, but this made up affair turns into reality because Iago wills it to be so. Surprisingly, in spite of his hatred towards Othello, Iago defends him from Cassio whom Iago thinks has cheated on Othello. He says...