Iago is using reverse psychology to convince Othello that he truly does not want him to know something
Iago is offering advice and wisdom on what it means to be a man who is cheated on, and Othello asks why he is saying this, where Iago will eventually convince Othello that Desdemona is cheating on him
That cuckold lives in bliss!
Why, why is this?
Iago convinces Othello that he wants to know what Iago has been thinking, and Iago begins to fulfill his plan from the beginning of the play, convincing Othello that Cassio is having an affair with Desdemona