How hard it is to convince a rock that it is hard? Or water that it is wet? Or a green-eyed monster that it is green? Or monstrous?
In Othello, Iago's plan to arouse Othello's jealousy is simple and easy. All he has to do is make an accusation: Othello will take care of the rest.
So says author and critic Salman Rushide:
She [Desdemona] didn't have to be guilty; the accusation was enough. The attack on her virtue was incompatible with Othello's honor.
Iago knows that Othello is paranoid of being thought a cuckold, a man whose wife lays her eggs in other birds' nests. It is the worst thing to call a man back then--worse than any racial epithet. In this sexist society, it is not difficult to portray women as unfaithful in a society where men think women are all unfaithful. It's not difficult to convince a man of jealousy when his is already jealous. It's not difficult to convince of husband that his wife is flirting when the husband is paranoid about it already.
Here's all Iago has to say:
Look to your wife; observe her well with Cassio;
Wear your eye thus, not jealous nor secure:
I would not have your free and noble nature,
Out of self-bounty, be abused; look to't:
I know our country disposition well;
In Venice they do let heaven see the pranks
They dare not show their husbands; their best conscience
Is not to leave't undone, but keep't unknown.
Iago's imagery is clearly visual: "look" and "observe." Men are arrogant enough to think they can read women's minds. And the Venetian society is built on distrust of women! All Desdemona has to do is laugh at Cassio's jokes, or touch his hand: Othello's jealousy is inevitable. The "green-eyed monster" isn't born in Act II or III--it has always been in Othello and other men of that time too.