Iago uses many ways of manipulation throughout the play to convince Roderigo to help get rid of Othello. One of the quotes is in the first act of the Play. Iago tells Roderigo,

"Thou art sure of me. Go make money. I have told thee often, and I re-tell thee again and again, I hate the Moor. My cause is hearted: thine hath no less reason. Let us be conjunctive in our revenge against him."(A.1,S.3,P.14)

 Here Iago is trying to convince Roderigo to help him bring down Othello. Roderigo is not a fan of Othello, considering the fact that he's married to Desdemona, and Roderigo wants Desdemona. One of the main reasons why it was so easy for Iago to psychologically manipulate him, is by convincing him that he only wants to get rid of Othello in order to help him Roderigo with winning over Desdemona, and that if he goes out and makes money, in exchange  Iago will get Desdemona to fall for him. (He's telling Roderigo to pay him for helping him get Desdemona to fall in love with him). 

In the real world, many people get manipulated by their so-called "friends" and lied to in order for their friends to fulfill their own desires. When someone is so in love like Roderigo, they don't really think about what they're doing. If someone tells them that this will help them win the girl, they'll do it. This also includes peer pressure (wanting to be accepted by a specific someone or a specific group of people, and sacrificing a lot to do so). In the novel, Roderigo thinks Iago is smart and important, and takes all the advice he is given from him and acts upon it thinking Iago is always right.