Your question seems to be in reference to Act 1; so I will answer it in accordance with Act 1. As you know, the play begins with Iago and Roderigo rudely awakening Brabantio and informing him that his daughter Desdemona has eloped with Othello. Brabantio is so furious that he tells Roderigo--whom he had banned from courting Desdemona--that he would have rather he (Roderigo) had married Desdemona.
After Iago's mission is accomplished in upsetting Brabantio and getting him to go after Othello, he sneaks off to Othello's to warn him of Brabantio's anger. When Othello hears Iago's warning, he responds with calmness and confidence. He knows his good reputation with the Duke of Venice and is sure that he will be able to explain why he married Desdemona and that they love one another. Of course, Othello's response is not what Iago was hoping for, because when Othello addresses the Duke, senators, and Brabantio and explains his and Desdemona's relationship, he is eloquent and convincing.
Shakespeare includes this detailed account of Othello's confident reaction to Iago's warning and Brabantio's rage because it contrasts sharply with the Othello that Iago turns into a wrathful and insecure husband later in the play.
This scene is why many critics argue that neither jealousy nor wrath is Othello's tragic flaw--because at the play's beginning, Othello is so confident in his reputation and skills that he is not a jealous or wrathful man. Rather, Iago uses his powers of manipuation to destroy Othello whose flaw his poor judgment in trusting Iago, a true lack of discernment.
I have wondered if Othello was ignorant or just so conceited that he could not see the truth in front of him. He has everything going for him, but he has this person who is jealous of him whom he has surpassed in position. He does not identify that the person would begrudge him his success. Othello has a loving and caring wife who seems to worship him, but instead of trying to find out the truth, he believes what he has been told. He attacks his wife with anger and does not really ask her if she had done anything wrong. He chooses to believe it and kills her. Because of his own ignorance, he loses everything.