In The Outsiders by S.E. Hinton, why does Dally tell Johnny that he shouldn't turn himself in?

Dally tells Johnny he should not turn himself in because he will not survive well in prison.

Dally comes to get Johnny and Pony and takes them to Dairy Queen.  Johnny tells Dally that they are going to turn themselves in, but Pony says that the idea must be a “jolt” to Dally.  Dally swears when Johnny tells him that he wants to turn himself in.

"I got a good chance of bein' let off easy," Johnny said desperately, and I didn't know if it was Dally he was trying to convince or himself. "I ain't got no record with the fuzz and it was self-defense. Ponyboy and Cherry can testify to that. And I don't aim to stay in that church all my life." (Ch. 6)

Johnny is afraid of going back because he is worried about cops, but he doesn’t think that he will get in much trouble.  He tells Dally that they will say that they hitchhiked because that way the cops won’t know that he helped them.

Dally decides to take them back, but they can tell he is upset.  He explains that the other boys in the gang are worried about them.  Johnny asks about his parents.  Dally tells him the gang cares, and that’s all that matters.

Dally asks Johnny why he didn't just turn himself in to begin with.  It would have saved a lot of trouble.  Johnny agrees.  He tells Pony that they ruined their hair for nothing.  However, Dally tells Johnny that he isn't the type that will survive jail.

" ... You don't know what a few months in jail can do to you. Oh, blast it, Johnny"--- he pushed his white-blond hair back out of his eyes--- "you get hardened in jail. I don't want that to happen to you. Like it happened to me..." (Ch. 6) 

Dally is right about the boys not going on the run in the first place.  If the boys had not gone on the run, they would not have hidden in the church.  The church fire is what caused Johnny to get hurt.  Johnny’s death caused Dally’s suicide.