In the play Othello, why does Iago hate Cassio so much (besides the fact that Cassio got the job Iago wanted)?
Yes, Iago resents Cassio because Iago resents the privilege that Cassio represents, and Iago equates this with a lack of manliness. We see this in the opening scene when he calls Cassio “bookish” and a “spinster” (1.1.23) while he himself, an experienced soldier, passed over—“And I, God bless the mark, his Moorship’s ancient!” This is also why his anger seems so excessive when Cassio acts mannerly with Desdemona, patronizing Iago as he does so: “Let it not gall your patience, good Iago, / That I extend my manners; ‘tis my breeding / That gives me this bold show of courtesy” (2.1.97-99). Iago cannot stand it. We see this “class feeling” again in Act 5 when Gratiano and Lodovico have forgotten his name: “these bloody accidents must excuse my manners / That so neglected you.” (5.1.93-95). Here he apes the “courtesy” that Cassio says Iago lacks (Act 2). Because he wants to be accepted as an equal, he addresses Cassio as “brother” ( 5.1.71). Therefore, his bitterness at Cassio springs not so much from disappointment over not being promoted on just this one occasion as from his general sense that he is not acceptable as “officer material.” He compensates for these humiliations, the daily beauty of privilege “that makes me ugly” (5.1.20).
I'd add to sagetreib's excellent answer.Cassio certainly has privilege and he gets what he wants without having to work hard to get it (Iago, in contrast, feels he has to scheme and work for everything he gets). He thinks Cassio has a "daily beauty" in him that makes Iago ugly by comparison. This beauty is his status, and also a general quotidian attractiveness or polish that Cassio possesses that Iago thinks he does not.
For all of his bravado, Iago seems to lack self esteem. He speaks so often of not being inferior to Cassio that we think he must surely feel it. He doesn't seem to have any confidence in what he does have, either, and in his mind, he thinks Cassio is the type of person who could steal from him that which he should have. Just as Iago believes the rumours that Othello slept with Emeila (Iago's wife) without having any evidence that it is true, he also believes that Cassio has slept with Emelia (with a similar lack of evidence). As far as Iago is concerned, Cassio has stolen his position, his security, and his wife. And, to top it all off, he's attractive.***
***If you are of the camp that questions Iago's sexuality, then Cassio's attractiveness adds another possible layer to Iago's hatred...
According to me, Iago hates Cassio as he got the position which Iago wants. Iago worked hard for that position and he considered himself as a good soldier and capable candidate for that position. According to Iago, Cassio is a book warm and a intelligent person more than him but he can't perform well in the battle field. When othello choose Cassio, Iago felt angry and vows to take revenge from them.