Roderigo's infatuation with Desdemona keeps Roderigo believing Iago's lies. Even from the beginning of the play when Roderigo is obviously exasperated by the fact that Iago has failed to set him up with Desdemona and that she has, in fact, eloped with Othello, Roderigo will not sever his ties with Iago. Iago promises him that he hates Othello too and that eventually the marriage between Othello and Desdemona will fail. She will "change for youth," Iago tells Roderigo, and that youth will be Roderigo, if he is patient. Roderigo believes Iago because he wants to believe him. Iago gives him hope that one day he and Desdemona will be together. We know Iago is lying because of his soliloquies:
Thus do I ever make my fool my purse.
But we must remember that Roderigo is not privy to Iago's thoughts. He only hears Iago's promises that if Roderigo can gather enough cash, Iago will make sure that he will eventually have Desdemona. Blinded by love, completely taken in by Iago, Roderigo is a fool. But as we see later in the play, Iago is certainly able to ensnare more powerful, smarter, and dangerous prey than Roderigo.