In Othello, there are many contrasts and confusion is caused when characters develop traits not necessarily expected of them. Othello, whilst a noble and decorated soldier, is no exception to this.
He loves Desdemona and, at first, finds it preposterous that anyone could suggest she feels any differently:
"she had eyes and chose me"
and refuses to believe that she would be unfaithful to him, demanding physical proof from the manipulating Iago. The more he thinks about the unlikelihood of Desdemona actually choosing him, the more he realises he must have been fooled:
He begins to act as if her unfaithfulness is a certainty.
The proof is presented to him after he has already started his downward spiral caused by jealousy and a lack of confidence.
"By heaven, I saw my handkerchief in 's hand"
The nagging doubt expressed in this language - long before the handkerchief is found - is enough to cause a character shift. He does so trust "honest, honest, Iago" that he cannot contemplate the possibility that Iago would lie to him.
Iago is almost using reverse psychology on Othello and there is a rise of panic in Othello's voice. His short, sometimes one-word questions and answers - "indeed"..."honest" - start the progression. As Othello is already susceptible to jealousy, he cannot recognize either his own failings or how he is being managed by Iago. Iago knows that Othello trusts him and that honor and honesty are paramount virtues. He capitalizes on Othello's susceptibility to the "green-ey'd monster" and having sewn the seed of doubt he
leaves him with his thoughts.
Anyone with paranoid thoughts will only misinterpret anything that follows. Iago is not in a hurry and wants to be sure that Othello (apparently) draws his own conclusions. He waits patiently for his opportunities and is then able to make the most of them. This is why his lies are all the more effective.