We're all wrapped up in our own heads. As far as life is concerned, it's our world and everyone else is living in it. We're the center of the universe and other people orbit around us — the distance of their orbit's circumference entirely dependent on how close we are to that person.
This isn't to say that everyone is a self-centered a**hole. Many of course care about other people, but the reality of being aware of yourself is that in a sense, you're writing your own story. When you consider others and how much they matter to you, it's that qualifier that's key — who they are in relation to you.