In Shakespeare's Othello, what does Iago see Cassio do that delights him?

In Act 3, scene 1, Iago derives great pleasure from seeing how Cassio behaves towards Desdemona. This happens shortly after their arrival on the island, Cyprus. In an aside, Iago declares:

He takes her by the palm: ay, well said,
whisper: with as little a web as this will I
ensnare as great a fly as Cassio. Ay, smile upon
her, do; I will gyve thee in thine own courtship.
You say true; 'tis so, indeed: if such tricks as
these strip you out of your lieutenantry, it had
been better you had not kissed your three fingers so
oft, which now again you are most apt to play the
sir in. Very good; well kissed! an excellent
courtesy! 'tis so, indeed. Yet again your fingers
to your lips? would they were clyster-pipes for your sake!

Iago comments on Cassio's courteous and gentlemanly behaviour towards Desdemona. He loves the idea that Cassio takes Desdemona's hand and whispers in her ear for he will weave a web around yhis behaviour to ensnare Cassio. He has formulated an invidious plan to implicate Cassio as having cuckolded Othello. He wants Cassio to smile at Desdemona for he will trap him in, as he believes, Cassio's own courtship. Cassio's actions will be enough to convince Othello that he is showing affection to Desdemona, further supporting the idea that they are having an affair.

Iago further says that if these actions are enough to make Cassio lose his position as lieutenat, then he would regard having kissed his fingers so consistently (a gentlemanly flourish in apparaisal and courtesy to a lady). All that Cassio is doing is being a gentleman and Iago wants to use these actions as a weapon against him. When Cassio repeats theflourish with his fingers and lips Iago says that Cassio would wish that his fingers were pipes which carry sewage for having implicated himself in this manner.

Iago's deep jealousy and resentment for Cassio is obvious in these lines. He hates him because Cassio occupies the position he so desperately sought. Furthermore, Cassio is a good man, a true gentleman and good-looking to boot. He has a certain virtuous and likeable air about him, which Iago despises.