Why do People think they know you

Why do people think they know you. Just because you work with people why do you assume that you know them when you really don't. I had this conversation today with a co-worker. Working relationships is one thing but to know someone outside of the Office is another. You can't really expect to know someone if you don't spend time with them. So once again how do people think they know you if they don't share your thoughts, ideals and goals.