This above all — to thine own self be true;
And it must follow, as the night the day,
Thou canst not then be false to any man. Hamlet, Act I, scene III
My Take - If you can't keep promises to yourself, how well will you honor promises to others? If you're not honest with yourself, can you be honest with the people in your life?
Conversely, if you are dishonest with others, can you be honest with yourself?