One of the overriding issues, and therefore a main theme of Othello is jealousy. From the onset, it is Iago's jealousy that drives him, that fuels his hatred for Othello and that will ultimately destroy Othello and cloud his judgment so severely as to destroy the one thing that is sincere and real - his love for Desdemona and her reciprocal feelings.
Iago stoops so low so as to encourage jealous feelings in everyone he encounters. He is not shy when he tells Roderigo how he hates Othello. Roderigo is angry that he has been rejected as a suitor for Desdemona and is willing to pay vast sums of money to secure her hand in marriage.
Iago, who never intended to help Roderigo, is so jealous of Othello and Cassio who he feels caused him to be passed over for promotion and who had no rights over his claim to be lieutenant that he tries to turn everyone against Othello, in fact, ultimately, against each other. Iago's need for revenge and his completely usurped picture of reality, sets the scene in Othello.
Iago mocks Othello and his supposedly high rank:
his Moorship's Ancient
reveals a sarcastic tone and his jealousy towards Othello's position as if Othello does not deserve it.
Love and hate, reality and appearance
I am not what I am
and the issue of jealousy itself all serve to intensify the power of jealousy when it is irrational and 'makes' men do things beyond any rational thought. It is not by accident that Iago is the first person we meet in Othello and who creates the false image of Othello as
an old black ram
He tries to create a false image of Othello even before we get to meet him. He schemes throughout purely motivated by this jealousy.
The eNotes pages will help you in understanding this main theme and the characters as they each contribute to it throughout Othello.