Why does Ponyboy imagine various worst-case scenarios while he is waiting for Johnny to return with supplies?

At the beginning of Chapter 5, Ponyboy wakes up alone in the abandoned church. He calls for Johnny, and when Johnny doesn't answer, he begins to panic until he looks down at the floor. On the floor of the church, Johnny had written in the dust that he had left to get supplies and would be back soon. While Johnny is out getting supplies, Ponyboy's active imagination begins to take over. Ponyboy remembers the night of the stabbing in vivid detail and imagines various worst-case scenarios. He imagines what it would be like if he had been sleeping for a week, and Johnny was picked up by the police, waiting to get the electric chair for not telling them where Pony was. He then thinks about what it would be like to die alone if Dally got into a car wreck and no one ever knew that he was staying in the abandoned church. Ponyboy mentions that he is feeling the delayed shock from the traumatic incident. He feels scared, traumatized, and hopeless sitting in the church alone, which is why he begins to think of worst-case scenarios. Ponyboy finally settles himself down and feels better when Johnny returns.