Some people are good at hiding their true feelings.They’re better actors than the average person. Others try to hide their feelings but have the kind of face that expresses emotion so plainly that they have nowhere to hide, nowhere to run to! And some people like to be open and aren’t so careful to hide their feelings.
So there are conscious choices being made about how much they want to divulge of their inner life. Then there are differences in how self-aware they are of their feelings and motivations and the meanings of their actions as Facade mentioned. An insightful person who is good at picking up non-verbal clues, and who watches and observes patterns of behavior can pick up on something that the other person is trying to block out from others or from themselves. As others have mentioned above denial and repression as well as other defense mechanisms interfere with self awareness.
When we feel confused and need to sort out our feelings, don’t we always go to someone who knows us well, but can be more objective than we can be about ourselves? But when we are not looking for the advice and it comes to us, we often resent and reject it. It’s only natural. We may not want to face the thing that is so readily apparent to others. Still, they see only outward signs without truly experiencing our inner reality. Am I contradicting myself? As usual, Wundayatta /you have asked a supposedly simple question that is actualy quite complex! And here I am tying myself up in knots trying to answer it, lol.
Bottom line:
Others can observe us and have insights, and conjectures about what our behavior means, but ultimately we are the one experiencing it and only we can know what that feels like. We cam’t avoid having others attribute meanings to our actions and guessing at or psychoanalyzing our motivations. In some cases they may be right about certain things. But to me, “knowing” involves more than putting a meaning to something. It involves the inner experience of your your own unique self, and defined as such, I would have to say, no, no one can know ourselves better than we do. One caveat, as far as mental illness goes I am no expert, but knowing your self and understanding, knowing and being objective about your behavior are two different things.
I hope I wasn’t too confusing with this answer, damn, I feel tired now