Othello has several reasons for not investigating the claims against Desdemona’s infidelity.
Firstly, he trusts Iago. He refers to his ensign constantly as ‘honest’, as do the other characters. He has no reason to distrust Iago, as he is unable to see the resentment which has built up within his trusted servant.
Iago is a master manipulator, and is able to skillfully manipulate his master in to wrongly reading events in front of him as sinister and indicative of deception. When Cassio is following Iago’s advice and petitioning Desdemona to convince Othello to forgive him for his drunken actions in Act III, scene iii , Iago draws attention to Cassio’s rapid departure at Othello’s arrival as symbolizing an intimate liaison –
Ha! I like not that
He then uses reverse psychology to manipulate Othello in to seeing something inappropriate in Cassio’s actions –
Cassio, my lord! No, sure, I cannot think it,
That he would steal away so guilty like,
Seeing you coming.
Finally, Othello does not really wish to believe that Desdemona has betrayed him. He has enough insecurity within himself for such an idea to gnaw at him, as of course it was predicted by Iago, who warned him of the ‘green –eyed monster’. He is used to deception and betrayal as part of his military occupations, and perhaps believes that Desdemona was too good for him. His fatal flaw is that he cannot make sound judgments in his personal relationships. As he says in his final speech, describing himself as
One who lov’d not wisely, but too well.