I'm also from Melbourne, and have lived in California for fifteen years. I put what you describe down to how often people move in America vs Australia. In Aust

I'm also from Melbourne, and have lived in California for fifteen years. I put what you describe down to how often people move in America vs Australia. In Australia, many people tend to stay in the same city for most of their lives, whereas America is full of people who go to high school in one state, university in another, and work in more - even those who stay in the same state often move cities (America has way more cities with way fewer people) they are constantly making new friends. This means things like: in my 20s in Melbourne, parties tended to consist of friends who've known each other for a long time. In America, the host often didn't know everyone at the party because the invitation is open. Canberra is actually a little more similar to America - most who live there came from somewhere else, so the social situation is much more open.