Since he is very flirtatious with women, it is easily believed that he would try to have an affair with Othello’s wife Desdemona due to his reputation. Iago uses this to trick Othello because he is too easily trusting and will believe what he is told without further thought. He also believes Iago to be an honest man and so he chooses to believe him over Desdemona. Roderigo believes that Iago will help him get with Desdemona and so he chooses to help Iago for his own benefit.
