Iago is trying to put these suspicions into Othello's head about Cassio and Desdemona. However, Othello is in love with Desdemona and could never think that she would cheat on him. Especially with Cassio.
"Excellent wretch! Perdition catch my soul. But I do love thee! And when I love thee not Chaos is come again."(3.3.90-92)
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"Did Michael Cassio, when you wooed my lady.Know of your love?"(3.3.95-96)
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"Give me a living reason she's disloyal."(3.3.417)
"I do not like the office... Cry" Cursed fate that gave thee to the Moor!"(3.3.417-433)
Iago tells Cassio what he has been seeing between Desdemona and Cassio and Othello is now starting to believe it and see the same thing. Although these accusations could be false, Iago's manipulation is changing Othello's view on Desdemona and Cassio.
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"Never, Iago... I here engage my words."
Now that Iago's words have manipulated Othello mentally, Othello now wants revenge on Desdemona and Cassio because of the accusations of their secret relationship.