This might be because probably we feel some insecurities when dealing with new people. This might be an instinct or something like that.
For example, animals also act differently when seeing another animal for the first time. We don't feel as comfortable interacting with people we don't know. I believe all of this is related to our instincts
I totally agree. It's almost always because of dealing with new people and to a great extent, the fear of judgement from these new people.