How does Iago enlist the aid and the trust of Roderigo in the first scene?

Iago is able to enlist Roderigo's help because of his feelings for Desdemona. Roderigo is in love with Desdemona and more than willing to take up the cry in informing her father she has run off with Othello. Iago makes full use of Roderigo's passion and feelings of dismay.