Final answer:
Iago is jealous of Cassio because he believes Cassio undeservedly obtained the role of Othello's lieutenant. Iago also suspects that Cassio had an affair with his wife, Emilia.
Step-by-step explanation:
Iago is jealous of Cassio due to a combination of professional and personal reasons. Professionally, Iago is resentful that Cassio was promoted to the position of Othello's lieutenant, a position that Iago believes he deserved. Personally, Iago suspects that Cassio has had an affair with his wife, Emilia. These feelings of jealousy and betrayal drive Iago to seek revenge against Cassio and Othello throughout the play.