Why does Johnny think he is a hero?
I agree with the previous post that Johnny does not consider himself a hero in S. E. Hinton's The Outsiders. Johnny was willing to rush into the burning church to save the children inside because he realized that it may have been his cigarette that caused the fire. He and Ponyboy saw that the chaperones were not willing to take the same initiative, so they went in after the kids. Dally followed in order to save Johnny and Pony. In the hospital, Johnny does not savor his new-found heroism; he is in too much pain, has been told that he is paralyzed, and he realizes that his life will never be the same
I do not really think that Johnny thinks that he, himself, is a hero. He certainly never actually says that he is a hero.
The only thing I can think of in the book that implies that he thinks he is a hero is when Pony comes and visits him in the hospital for the first time. Pony tells him that they are being called heroes. Johnny says "tuff enough' and looks proud.
If he truly thinks he is a hero, then it must be because of the fact that he risked (and eventually gave) his life in order to help the kids escape the fire.