Iago claims he hates Othello because Othello passed him, Iago, over for a promotion, giving "one Michael Cassio" the job as his military lieutenant instead. Iago claims hes far more qualified than Cassio, who lacks Iagos experience on the field of battle.
One Michael Cassio, a Florentine,A fellow almost damnd in a fair wife;That never set a squadron in the field,Nor the division of a battle knowsMore than a spinster; unless the bookish theoric,Wherein the toged consuls can proposeAs masterly as he: mere prattle, without practise,Is all his soldiership. But he, sir, had the election:And I, of whom his eyes had seen the proofAt Rhodes, at Cyprus and on other groundsChristian and heathen, must be be-leed andcalmdBy debitor and creditor: this counter-caster,He, in good time, must his lieutenant be (1.1.2)
The lovesick Roderigo has trouble with his hidden feelings for Desdemona and is jealous watching the two in love. Because of his love for Desdamona and subsequent jealousy of Othello, Roderigo readily agrees to help Iago in his plans to bring down Othello.
Bianca, Cassios mistress, also becomes sick with jealousy. She sees the planted handkerchief in Cassios room and believes him to also have taken a lover. Her jealousy exists on a much smaller scale, but illustrates that the sentiment is universal.
Othello is easily taken in by Iago’s lies and manipulation because of his age, race, and because he loves “too much”. When Iago tells Othello of the handkerchief, he has the evidence necessary to prove Desdemonas unfaithfulness. It is now that the jealousy sinks deep into Othellos soul and starts to destroy his mind.
I pray you, in your letters,When you shall these unlucky deeds relate,Speak of me as I am; nothing extenuate,Nor set down aught in malice: then, must you speakOf one that lovd not wisely but too well;Of one not easily jealous, but, being wrought,Perplexd in the extreme; of one whose hand,Like the base Indian, threw a pearl awayRicher than all his tribe; of one whose subdud eyesAlbeit unused to the melting mood,Drop tears as fast as the Arabian treesTheir medcinable gum. Set you down this;And say besides, that in Aleppo once, O! now, for everWhere a malignant and a turband Turk Farewell the tranquil mind;Beat a Venetian and traducd the state, farewell content! (3.3)I took by the throat the circumcised dog,And smote him thus. (5.2)