It is especially hard, but extremely important, to be honest with regards to the stock market and your trading. First with yourself and then with others. If you are not honest with yourself, you will never know the real reason you make mistakes and you will never be able to correct them. If you are not honest with others, well.... if you are not honest with others then I don't think you are really being honest with yourself.
It is really hard to be honest with trading because the market often makes you look like a fool.
I think a lot of it has to do with confidence and determination. Confidence that you are not a fool even though you may often look like one and determination that you will succeed even though you may many times fail.