Some people go too far. That's because for all their lives, people have let them go too far. What's up with that though?
Do they feel like they have the right to act a fool when they don't get what they want? Do they think that what they are doing is cute? Have they been able to act this way and get away with it with everyone they know? Who has been letting them do this stuff? Has it been cool, or cute to them?
Can someone become a brat after the fact? Can you be used to not getting your way, but then work for what you want and then act a fool when you don't get something because you worked for it?
Can you live a life of not being selfish all your life and then become selfish for some reason?