It's one of those beliefs so widely agreed on as to be beyond question: You are the exclusive expert on you.
Being an expert tends to go along with not being so open to what other people have to say. And when it comes to ourselves, being an expert on you tends to mean being the expert on you.
But the fact is, there's a whole lot about you that you're in the worst position to see. Like how you're perceived, how you do relationships, whether you're really such a good dancer--all sorts of things that just can't be seen so well from your own vantage point.
If you're open, and you really listen, people will tell you some things about who you are. Often it will be unpleasant. Occasionally it will contradict your own, expertly held beliefs on the matter.
One of many things I love about group therapy is that after a while, you have a whole new panel of experts on you: They know your strengths and your weaknesses. They know your tricks and cover ups. They know the ways to tend to sell yourself short. They've seen you shine and seen you falter. And that has tremendous value when times are tough, or you have to make a hard decision, or you want to make a big change.
One of my group therapy patients characterized it like this: It's kind of like having somebody tell you that you have food stuck in your teeth, but for really important life stuff.