Why does Othello trust Iago?
Othello trusts Iago because he has a noble spirit and assumes that other people are like himself. He is like many other Shakespearean characters who become disillusioned with humanity when they learn the truth about human nature. Iago is an exceptionally cunning man and is capable of fooling almost anybody. Furthermore, Othello would have no reason to distrust a man who was working for him and dependent upon him for his livelihood. Iago is also able to hoodwink Cassio, Roderigo, Desdemona, and his own wife, showing his Machiavellian cunning. Othello is very much like Brutus in Julius Caesar. Brutus trusts Cassius, a cunning, greedy man who is very much his inferior, until they have a falling out. In Shakespeare it is common for people of noble character to trust others because they unconsciously assume that others are like themselves. King Lear is an example; he trusts his two wicked daughters. Prospero in The Tempest is another example. The villains, on the other hand, also assume that others are cunning, selfish and dishonorable like themselves. The fact that Othello is a complete outsider to Venetian society is also significant. He simply doesn't understand the mendacity of the people he works with. He also respects Iago because his ancient has shown great courage in battles, as Iago describes his military services to Roderigo in the opening act of the play.
Shakespeare has portrayed almost all of his great tragic heroes i.e. Macbeth, Othello, Hamlet and King Lear and so on, with a severest kind of fault in their nature which is called hamartia or tragic flaw. As such we can assume that Othello can also be not an exception to this hamartia. In othello's case this massive flaw seems to be his credulity. othello, the tragic protagonist, relies completely on Iago. And hence this Machiavellian villain remains always active to shape out his serpentile schemes by the way of Othello's too-trusting temperament for the destruction of the latter.
We can see that as a consequence of some unexpected events, in the play, Othello becomes enraged with Cassio and also with Desdemona; even he loses faith in her whom once he had loved and trusted so dearly. But in the case of Iago he does never lose faith. Every single word spoken by Iago gains trust and support by the tragic protagonist.
The case can also be seen from a different point of view. Earlier Brabantio had warned othello about Desdemona that the girl who betrays her father can also her do the same with husband, at the court, while Desdemona expressed her decision to marry Othello. These words of Brabantio might have left some negative effects in Othello's unconscious mind and othello who is sure to be a naive person by nature gradually shifts his entire faith to Iago, the villain with the 'motiveless malignity'.