Why does johnny look like he is having the time of his life in the church secene?

Johnny looks like he is having the time of his life because he is away from most of the things that make him afraid. He is not staying at his abusive home. He isn't walking the streets, open to the Socs' beatings, and he has a 'brother'. When he gets the chance to be a hero and save the kids in the building he feels as though he life has meaning. He is able to do something good.