What makes people care about others behind their backs?

If I am with someone in person or talking over the phone, they are engaged and care about me. But what happens when I am not with them? Why would anyone care about me when I am not around them? What makes people care about you behind your back? What makes people care, at all? Essentially why anyone would do anything without any reason, why anyone would unconditionally care? Little Background: I have never been a compassionate person, I don't understand compassion. (edit) Or like why would a Buddha care about me? I provided nothing to them, they even never met me, but I feel still they will care about me. Why? How? (will this question suit better in any other StackExchange?)