How do you know if you really know someone?

I was told yesterday that the best thing about me is I am not afraid to make a person feel special. That I am not afraid to put myself and my feelings out there. I am not afraid of being hurt. That I am the kind of person that will not let an opportunity pass. That I will tell you how I feel because not telling you will hurt me so much more.
I was told this by one of the few people I feel really understands me. He is the type of guy that always knows what is going on with me, sometimes even before I can figure it out. Talking to him sometimes scares me, because sometimes I feel like I do not want a person to understand the feelings I keep inside. The reactions and thoughts that I do not completely agree with that I try to suppress in an attempt to control. But he understands them all… and he accepts them.
Sometimes I feel like he sees me in a light so much more attractive than the one I see myself in. That when I am harsh on myself and not accepting of my flaws he convinces me that I am a good person and I deserve a break from my harsh judgments.
He just makes me wonder how a person can understand me better than I understand myself. How can a person see so deep inside of me? See the parts of myself that I keep hidden from everyone, including myself… especially myself. Why can I not see as clearly as he can?
If I do not know myself is it possible to actually know anyone else? Do I know myself? Does he know me better? Or do I just believe him because it sounds so much better than my harsh judgment?