Iago absolutely hates Othello. He is angry with Othello for promoting Michael Cassio, an officer with no real battle experience, to lieutenant over him. Once Iago comes up with a plan for revenge it does not take very long for him to set it into motion. Iago's plan is to cause Michael Cassio to fall out of Othello's good graces. Once Cassio has lost his position as lieutenant Iago knows that Cassio will do anything to get it back, so he advises Cassio to get Othello's wife, Desdemona, to speak on his behalf to Othello. Iago tells Cassio, "Our generals wife is now the general... Confess yourself freely to her. Importune her to help to put you in your place again." Iago has begun his plan to poison Othello's brain.
Once Desdemona begins defending Cassio to Othello, Iago begins to make Othello think that there is something odd in the situation and that Cassio and Desdemona are having an affair. Iago does this because he knows that Desdemona is one of Othello's only weak spots. Othello trusted Desdemona implicitly "my life upon her faith"- showing the audience the extent to which her faithfulness rules his life and his trust. Iago knew Othello loved her so much he would do anything for her, and if he could destroy that love in some way his plan would work. Iago is deliberately plotting for Othello's downfall.
Othello failed to recognize the strength of this love when Iago started to put the negative thoughts of infidelity into his head. He started to believe Iago right away, and he seemed to forget how much Desdemona had already proved