’Tis my breeding
That gives me this bold show of courtesy. (2.1.)

Cassio makes this comment to Iago after greeting Emilia with a kiss. He intends to signal that he did not mean any disrespect by kissing another man’s wife but that this sort of behavior is simply part of the good manners he is used to displaying. He might also be commenting on manners in his native Florence being different from what would be expected in Venice. The quote is significant because it shows how Cassio’s gallant and possibly even flirtatious behavior could be open to misinterpretation, a weakness Iago will later exploit.

An inviting eye---and yet methinks right modest. (2.3.)

Cassio compliments Desdemona to Iago as the two men praise Desdemona’s beauty. Iago seems to be trying to get a read on Cassio’s feelings about Desdemona by encouraging him to praise her. Cassio certainly acknowledges her beauty, but his comments remain respectful and he notes that Desdemona is a virtuous and loyal woman who always behaves appropriately. The quote shows that Cassio is not looking to make trouble in any one’s marriage, or cause problems for Desdemona.

I have very poor and unhappy brains for drinking. (2.3.)

Here Cassio explains to Iago why he needs to carefully moderate how much alcohol he consumes, since he has a weak tolerance and rapidly becomes very inebriated. Cassio thinks Iago is a trusted friend who will look after him and make sure he does not get into trouble. Iago, however, deliberately uses this information against Cassio, getting him very drunk and setting him up to be involved in a fight.

Oh, I have lost my reputation! I have lost the immortal part of myself and what remains is bestial. (2.3.)

Cassio says these lines in shame and sadness after Othello has angrily chastised him and stripped him of his position. Although not physically hurt, Cassio knows he has lost his public image and the respect of Othello, both of which pain him. The quote shows how much value Cassio places on his honor and reputation, and how he would never willingly do something shameful.