Because Iago is very good at being deceptive, he convinces Othello that his wife has been unfaithful with Cassio. Truly, Iago is a intelligent, gifted man. He understands what motivates the characters he manipulates.
Because Othello and Iago have been warriors, side-by-side, Othello trusts Iago:
Iago is a soldier with a good deal of experience in battle, having been on the field with Othello at both Rhodes and Cyprus.
No doubt, Iago considers Othello, the Moor, as a man who can easily be fooled and led around as one would lead an ass:
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.
Truly, Iago is sure of himself. Iago begins his evil plot by planting seeds of jealousy in othello.
To further his evil plot, Iago brings up the fact that Desdemona has been dishonest before. Iago reminds Othello that Desdemona lied to her father in the beginning. He reminds Othello that Desdemona did not tell her father the truth about her marriage to Othello:
She deceived her father by marrying you;
When Othello thinks about Iago's accusation that Desdemona was dishonest with her own father, Othello begins to suspect that Desdemona may be dishonest in other areas of her life. Iago plants seeds of doubt in Othello's mind and heart. Othello is afraid that Desdemona may lie about other things as well as she lied to her father about her secret marriage to Othello.
Of course, Iago quickly assures Othello that he did not mean to upset him. He realizes that Othello is clearly upset at the thought of his precious Desdemona being capable of lying. Iago realizes that his plan is working. To protect himself, Iago reminds Othello that he truly loves him. Iago is an excellent actor:
My lord, you know I love you.
Then Iago continues to assure Othello that he only created suspicion about Desdemona due to his true love for Othello:
I hope you will consider that what I have spoken
Comes from my love; but I see you're upset.
Othello believes every word Iago says. Othello thinks Iago is very honest and trustworthy:
Othello calls him honest, honest Iago (V.ii.155),
This why Othello turns against his beloved Desdemona. Also, Othello has self-esteem issues. He feels that he is much older than Desdemona. He also mentions that his skin color, which is black, may have a negative effect on his lovely wife. Truly, Othello is not secure in his position as Desdemona's husband. He doubts that she could love him for various reasons:
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,
Iago plays on Othello's low self esteem. He uses Othello's lack of confidence to further convince him that Desdemona has been unfaithful to him. Iago is crafty, sly, and very tenacious. Iago is a clever, cunning manipulator. He knows Othello's weaknesses and he uses this knowledge as an advantage over Othello.
Clearly, Iago plays Othello for a fool. Ultimately, Othello murders his beautiful, innocent wife. Iago wins.
In the end, Emilia convinces Othello that Iago has been behind the whole evil plot of destroying Othello's marriage. When Othello learns the truth about Iago, he realizes he has smothered his innocent, loving wife. Othello cannot live with himself. He stabs himself, thus committing suicide.