There's a lot of pressure on famous people to always be on their best behavior as they never know what private information will end up becoming public gossip. Interrupting a celebrity's dinner to start up a conversation about your fandom is pretty rude, and it isn't an actor or musician's job to entertain you on their "time off." Even though they live in luxury mansions and fly in champagne-filled private jets with bowls of only red M&Ms, they are people too (just like us!) and deserve an uninterrupted night out every now and then.
While everyone has a bad night and sometimes you might catch your favorite comedian or actress in a horrible mood as they wander through the frozen sections of the grocery store, there are some people who are so consistently rude (or shockingly friendly) to their fans or staff that they develop quite the (cough cough Ellen DeGeneres) reputation.