Why does Johnny not want to see his mother?

Johnny says firmly, "I don't want to see her." He says to tell his mother "For once just to leave me alone," then he passes out from exertion. The nurse tells Ponyboy he has to leave, and turns Two-Bit away as he arrives with the copy of Gone with the Wind. As they walk back down the hospital hall, they see Johnny's mother complaining that she's not allowed to see him. When she sees them, she tells them it is their fault that Johnny was hurt. They feel terrible that Johnny has to "live with that," and Ponyboy hopes that the nurse will do what Johnny wants and not let his mother in to see him.