Why are some patients so rude?

In the medical field as a receptionist you try to patients in every way.  I don't know if it is entitlement or just plain rudeness.  Many times we go out of our way to help patients out and some can't even say a simple thank you.  Does this have to do with narcissism?  We can't always get what we want and some patients just don't understand that if they don't do what is asked of them we can not accommodate them properly.   Susanne Reuner