Think about your real friends. People you'd hang out with when you're in almost any mood. People you'd vacation with. People you'd love to work with if given the chance.
Now think about a contrasting group. I call them “cocktail party friends”. They are the kind of people you meet that would be great to hang out with at a cocktail party, but you don't want to interact with them any deeper than that. They are great for 10 minutes at a time, interesting, funny, smart, quirky, and make for stories or information you can take away from the conversation.
Have you ever had a disconnect, where someone you know thought of you as a real friend, but you thought of them as a cocktail friend? And let's not forget the more unsettling thought, do any of your “real friends” see you that way?