I wrote about that for my essay. Iago mentions a lot of motives, such as the 'suspicion' that Othello has had an affair with his wife, or jealousy at having been passed over for a job promotion, or just plain racism, but it's obvious that he's really trying to justify to himself his hatred. I don't think any of the cited motives are true. I personally think Iago loves Othello, and maybe he's trying to deny it/can't gain happiness so tries to destroy the happiness of the object of his devotion. If you've seen the movie adaptation with Laurence Fishburne and Kenneth Branagh (<3), it becomes sort of obvious. The scene where they make an oath to each other, to kill Cassio and Desdemona, is so resonant with a homosexual theme that it's sort of ridiculous: