why did othello didn't believe Desdemona?

Othello saw things in extremes. Either Desdemona was pure and chaste or she was loose and disloyal. There was no in-between. This "Madonna- whore complex" was frequently seen in Shakespearean male characters. Once evil Iago planted the seed of discontent in Othello's head, the paranoia came easily. Othello refused to listen to reason rather he succumbed to his rash nature. Othello felt he was made a dreaded cuckold and nothing else would change his mind.