Answer:
It is the unluckiest day of Kevin Brown's life. He looks sad and upset because he is about to go to the dentist. He shows his teeth to his parents, who are going with him. They are sad because they think he looks sad. They know that the dentist is going to be disappointed that Kevin's teeth aren't straight.
It is the saddest, most miserable moment of Kevin Brown's life. He is getting new braces. At least at the dentist's office, they are ready to provide options but Kevin was sure we would also provide fillings. At least, they would have to finish the filling first.
Kevin begins crying because he knows he is about to face an angry and bored dentist. He is sad because it is not his fault; or rather he is not ready to cry. There was no way he was going to have to face the dentist without filling his teeth. It was not a single escape.