Othello’s fit was brought on as a result of him being unable to reconcile two things; the first being his deep love for Desdemona and his belief that she neither would nor could have any reason to betray him, and secondly, all the mounting “evidence” that a loyal friend was presenting to the contrary.
Othello really did love Desdemona and saw all the sacrifices she had to make (running away from home, disobeying her father, getting married in secret, going against cultural stereotypes, and choosing to go along to war with him) as affirmations of her love for him. With all the things she had to give up in order for the relationship to be possible you could argue that she was more invested than Othello, but either way everything she did and said only strengthened his belief that she had no reason to lie to him.
That what Iago does is immoral or unethical, doesn’t change the fact that he is a great manipulator, to the point were it is almost admirable.
Othello couldn’t possibly be more sure of Desdemona’s love for him and Iago manages to convince him of the opposite. He does it in such a way that Othello has no reason to believe that Iago has any ulterior motive to tell him this except out of loyalty and genuine concern for a friend.
Othello starts off completely sure of his convictions but Iago manages to squeeze in enough doubt into Othello’s head that he starts thinking not as he usually does but rather from the perspective that Iago wants him to think. He actually manages to make Othello feel insecure, which is in no way easy considering that; Othello comes of royal blood and has fought in life threatening battles and triumphed (he lived through them and wouldn’t have had the respect nor honor among the military if he hadn’t fought valiantly).