It means that telling other people that what they think and feel is less important because you don't agree with it really makes you look like an ass. Just because you think me wanting a cheeseburger isn't important doesn't mean other people don't feel the way I do. So you are a brat for butting in with unhelpful opinions.
Girls happen to find that very unattractive. Smacks in the face are standard responses to insensitive guys.