Why should Roderigo pay particular attention to Iago's speech?
Assuming that you are referencing Iago and Roderigo's conversation in Act 1, Scene 1, Roderigo should closely follow Iago's comments regarding what he plans to do to Othello because they demonstrate not only how Iago will seem to follow Othello even as he is betraying him, but the villain's words are also a warning to Roderigo that Iago serves only himself and could be putting on a facade when he is dealing with the "love-stricken" Roderigo. Iago even admits:
" 'I am not what I am' " (1.1.63).
Unfortunately, Roderigo, like most of the play's other characters, falls prey to Iago's masterful manipulation and acting until it is too late to save his own life.
Rodrigo is being used by Iago to incite Othello's jealousy. He tells Othello that Desdemona has given a handkerchief to Rodrigo that Othello had given to her as a wedding present. Iago gives the handkerchief to Rodrigo’s wife and it gets to Rodrigo, so it looks like Desdemona indeed has given it to the naïve Rodrigo and is unfaithful to Othello. Neither Desdemona nor Rodrigo are aware at all of Iago's evil machinations and are therefore powerless to defend themselves.