I have been fortunate this past month to have been working at the Bipolar Clinic at UCSF. Chief of the clinic, Dr. Descartes Li, shared with us this article that is one of the best discussions about why it is that people who are clearly manic are unable to recognize the change in mood.
It also has some useful suggestions for how to deal with this dilemma, and the comments are worth reading as well.
This is also why the development of smartphone apps that mood chart might be so helpful, they could provide objective evidence that there has been a change in behavior.
http://www.bphope.com/my-world-tricked-by-mania/