You never really know anyone. Even if you are the closest friends in the world, there are still things your friend won't tell you. There are things that you don't know, and never will. The only person you really know is yourself. You're the only person that can see into your mind, that can open your own memories. There is not another being in the world that can see into your head. Usually.
Be careful who you trust. The wrong one could rip open the parts of you that were always meant to be hidden. The parts that no one else was ever supposed to know.