I think that Othello can be seen as a fool because he does not limit his own sense of doubt and insecurity. This becomes the critical element in his own character that ends up causing him to believe Iago's machinations as well as believing in the very worst in Cassio and Desdemona. More importantly, Othello is foolish because he allows himself to believe the very worst about himself. He believes that someone like Desdemona could not love him because of the fact that he is not of traditional stock in terms of those who hold power. Being a warrior and a man of color, Othello views himself differently, something that enables him to see himself as someone who is not worthy of the merits that he has accomplished. Othello's self doubt and his own lack of confidence in himself is what makes him so foolish to believe the very worst about both himself and those around him. Othello's foolishness is something that reflects the very worst about himself and this is what causes him to believe the very worst others. It is here where his foolishness is most evident.