Final answer:
Iago's complaint in Scene I originates from his resentment towards Othello for promoting Cassio over him and his suspicion of Othello's involvement with his wife, Emilia.
Step-by-step explanation:
Iago's Complaint Against Othello
Iago's complaint in Scene I is rooted in his bitter resentment towards Othello. The primary reasons for Iago's hatred include Othello's choice of Cassio as his lieutenant over Iago, who believes he was more deserving of the promotion.
Additionally, Iago suspects that Othello has been romantically involved with his own wife, Emilia, fueling his animosity. Iago's complex character is driven partly by these feelings of betrayal and suspicion, leading him to manipulate and deceive other characters throughout the play.