Answer:
Iago creates the tale of Desdemona and Cassio's involvement by convenience. Iago so desperately wants Othello to become insane with jealousy, and he initiates this envy and rage by creating lies of Desdemona's unfaithful- ness. The chance comes when Iago and Othello see Cassio and Desdemona together.
IAGO: "O, beware, my lord, of jealousy; It is the green-eyed monster which doth mock the meat it feeds on;"(3.3. 15). Here Iago manipulates Othello. He tells him not to be jealous but at the end jealousy ends up destroying the heart of the man who falls prey to it.
