I think that a case can be made that the theme of jealousy runs rampant in Shakespeare's drama. Jealousy emerges when individuals have difficulty accepting the limitations and conditions of the world around them. Rather than embrace the reality of what is, characters infected with jealousy conspire and believe that they have been wronged. Oftentimes, the irrational and intense conditions of anger present are results of jealous behavior.
I think that one interesting instance of jealous behavior can be seen in Brabantio. Believing that he is isolated and protected from the difficulties of the world, Brabantio is stunned to learn that his daughter has gone off with Othello: "What tell'st thou me of robbing? This is Venice; / My house is not a grange." There is a shock that Brabantio expresses about the "wrong" that has been done to him. This manifests itself into a type of jealous behavior directed at Othello. Brabantio is jealous over how Othello understands his daughter better than he does: "Fathers, from hence trust not your daughters' minds
By what you see them act." It is inconceivable that his daughter would go against his own will. Yet, it is in this reality that he recognizes Othello better understands his daughter than he does. It is for this reason that he never sanctions the marriage and pleads to the Senate to strip Othello of his title. In his shock and dismay at failing to understand his own daughter, Brabantio displays jealous tendencies towards Othello.
Certainly, Othello displays jealous behavior, as well. Othello is so enamored with Desdemona and simultaneously so insecure about her love for him that he becomes an easy target for jealousy. This can be seen in the text through his own eyes. Othello speaks to his own jealous condition. At one point in Act III, Othello speaks to how he could not be jealous of Desdemona without "proof:"
Why, why is this?
Think'st thou I'ld make a life of jealousy,
To follow still the changes of the moon
With fresh suspicions?
Othello's display of jealousy is something that he believes he can repel. Yet, it is there, lurking underneath the surface. Othello demands that he will not acquiesce to jealousy without proof. In acknowledging its presence, Othello ends up displaying its tenets. Iago understands this and focuses his energies on the force of jealousy throughout his manipulation of Othello. At the end of the drama, Othello speaks as a man who has sadly embodied the theme of jealousy:
Speak of me as I am; nothing extenuate,
Nor set down aught in malice: then must you speak
Of one that loved not wisely but too well;
Of one not easily jealous, but being wrought
Perplex'd in the extreme
Othello represents the theme of jealousy and how it can undermine everything within a human being when left unchecked. Othello succumbs to jealousy because he is unable to accept the conditions of the world around him, internalizing the negativity of the world in his love for Desdemona.
When discussing how negativity is absorbed and replicated within the individual, one cannot go far without discussing Iago. Certainly, Iago embodies jealous behavior. He is jealousy incarnate. His inability to accept the conditions of the world around him cause him to be jealous. He is jealous because of Cassio's promotion. He cannot reconcile the fact that someone like Cassio would receive a promotion when he perceives himself to be so much more worthy:
Preferment goes by letter and affection,
And not by old gradation, where each second
Stood heir to th' first. Now, sir, be judge yourself
Whether I in any just term am affin'd
To love the Moor.
Iago is jealous for what Cassio has gained. Calling him "mere pratter without practice," a limited soldier, and a "counter- caster" are ways in which Iago expresses his jealousy because of the inability to accept the conditions of the world around him. In this case, it is Cassio's promotion.
At the same time, Iago is jealous of Othello. The insecurity that is within Iago has been externalized with Cassio's promotion. Directing this insecurity at Othello is another representation of Iago's jealousy: "I hate the Moor:/ And it is thought abroad, that 'twixt my sheets/ He has done my office: I know not if't be true;/ But I, for mere suspicion in that kind,/ Will do as if for surety." Iago is jealous at the mere hint that Othello has been involved with Emilia. Iago concedes that he does not know if the rumor is true, but it becomes clear that jealousy has already taken a hold of Iago without evidence. Whereas Othello needed evidence, jealousy was emerging within him. In Iago, the behavior is extrapolated and evidence is secondary. Iago represents the theme of jealousy throughout the drama.