Answer:
Iago hates Othello because Othello passed him over for a promotion, and gave the lieutenant job to Michael Cassio. This provokes Iago's jealousy, and throughout the whole play, he uses other people's weaknesses to achieve his great goal - to take revenge on Othello.
This accusation is a clear example of dramatic irony - Iago accuses Cassio of putting on a false appearance to gain his own ends, while he is the one who actually deceives others all the time.