In Othello, Iago easily deceives the main character Othello. One reason Iago can so easily deceive Othello is due to Othello's trusting nature. Othello truly believes that Iago is an honest man. In Act II, Scene III, Othello makes the following comment about Iago:
Iago is most honest.
No doubt, Othello is too trusting. Iago testifies to this in Act I, Scene III. Iago comments on Othello's free and open nature. He compares Othello to an ass which can easily be led by the nose:
[Othello] the Moor is of a free and open nature,
That thinks men are honest that only seem to be so;
And will as tenderly be led by the nose
As asses are.
In the beginning, Othello seems to trust Desdemona. He seems to believe that Desdemona truly loves him and supports him. Then Iago plants seeds of doubt in Othello's heart and mind. He accuses Desdemona of having an affair with Cassio. Next, Iago subtly points out that Desdemona is capable of lying. He points out that she lied to her own father about her marriage to Othello:
She deceived her father by marrying you;
Iago's comment causes Othello to doubt his lovely wife. Othello truly trusts Iago for Iago keeps assuring Othello of his love:
My lord, you know I love you.
Othello believes Iago's empty words. Again, Iago deceives Othello with his mere words. He apologizes for loving Othello too much:
I humbly beg your pardon
Because I was loving you too much.
Iago is saying that he had to express the truth about Desdemona because he loves Othello so much. Othello responds that he is forever indebted to Iago for Iago's honesty in sharing the truth about Desdemona:
I am bound to you forever.
Clearly, Othello is convinced that Iago is honest and truthful. Iago realizes that Othello is upset by Iago's insinuation that Desdemona is dishonest. Othello is thinking that if Desdemona will lie to her father, she may lie to him. Iago uses mere words to deceive Othello while professing his love:
I hope you will consider that what I have spoken
Comes from my love;
Truly, Othello is easily deceived. Also, no doubt, Othello has self-esteem issues. Because of his own insecurities, it is easy for Othello to believe that Desdemona would be unfaithful. Othello states his reasons that Desdemona would be unfaithful:
Maybe, because I am black,
And don’t have those soft parts of conversation
That gentlemen have; or because I am getting
Much older, still that's not much,
She's already unfaithful; I am abused, and my relief
Must be to hate her. O curses on marriage,
Clearly, Othello is not secure in being a black man. He even uses figurative language to insinuate that he is insecure in being black. He compares Desdemona's unfaithfulness to his own black face:
Her name, that was as fresh
As Diana's face, is now as grimy and black
As my own face.
After making the comment that Desdemona's face is as grimy and black as his own, it is clear that Othello is not happy with being black.
Also, Othello shows how confused he is. He wants to believe his wife. He admits that he is not sure what to believe. He is divided in his heart. He admits to Iago that he is torn in his mind on who to believe or what to think:
I think my wife is honest, and think she is not;
I think that you are just, and think you are not.
I'll have some proof.
Because Othello is emotionally unstable during Iago's scheme, it is easy for Iago to deceive him.