The truth is we will never know another person really well, especially since we never really get to know ourselves all that well either. I can predict my own reactions or behaviour, but I'm still discovering things about myself and I have no reason to believe that this process of self-discovery will end any time soon. It's logical to assume that since we spend most of our lives discovering ourselves, we never really have the time to get to know other people well. We think that we do, but that's a whole different thing altogether.