He trusts the wrong man...Iago. He will not listen to others around him that try to tell him the truth. It is in part his own lack of self confidence and low self esteem that allow Iago to so completely manipulate him.
He is upset that he's been passed over for promotion, and he's upset that a Moor is sitting in a high ranking position. He also suspects that Othello has had an affair with his wife. He also may have secret yearnings for Desdemona, himself.
Act 1, Scene 1, first early lines.