Act 3, scene 3

Iago questions why Cassio should "steal away so guilty-like / Seeing your coming."

Beware of jealousy, my lord! It's a green-eyed monster that makes fun of the victims it devours. The man who knows his wife is cheating on him is happy, because at least he isn't friends with the man she's sleeping with. But think of the unhappiness of a man who worships his wife, yet doubts her faithfulness.