Why does the narrator insist that he's not a madmad

This is mostly because the narrator is rather crazy. From his beginning words, we can tell that the narrator is unreliable, "TRUE! --Nervous --very, very dreadfully nervous I had been and am; but why will you say that I am mad?" As the story unfolds there is very little of this man that we can take at face value. The guy wants to kill his roommate over a perceived bulging eye. Poe must hence have his narrator insist on his sanity. This adds even more doubt to this already unstable mind. The narrator's repeated insistence to his sanity is both comical and interesting.