Final answer:
Iago dislikes Othello because he was passed over for promotion, choosing Cassio as lieutenant instead. This fuels a vengeful plot central to the play's tragedy.
Step-by-step explanation:
In William Shakespeare's play Othello, Iago expresses his hatred for Othello primarily because he feels wronged when he is passed over for promotion. Iago is resentful that Othello promoted Cassio to be his lieutenant instead of him. This is a central motive for Iago's manipulative actions throughout the play. While prejudice exists in the society they inhabit, Iago's personal vendetta is driven by his ambition and need for recognition, rather than racial animosity or a desire for Desdemona. His complex character is fueled by a mixture of envy and perceived personal slights.