You're supposed to put yourself into another; that's why when someone dies or goes away, etc. you are sad and grieve, you grieve for that piece of you you invested (psychological term I think) in the other. You have to get your investment "back". The other person is a mirror for you and you behave according to what you see and all that.

However, if you don't know yourself, what you like, dislike, how to center yourself, calm yourself, be "alone" with yourself, etc. then you would have a hard time knowing what it was you were seeing in the mirror of the other person? So, when they show something you don't "recognize"/want in yourself, you're going to reject them? It's only knowing what you're capable of and "living" with all the aspects of yourself a bit that you realize that whatever you see is "you" and okay. It's like when you look in a funhouse mirror. They're usually not scary (or even particularly interesting) to adults because adults know what they look like but children haven't seen or imagined themselves in different ways before so they're fascinating to a kid?