What was the reason Ponyboy told Randy on why he went back into the church to save the kids?

From the text, we can infer it was pure instinct.... just something he had to do because it was the right thing to do.

"I read about you in the paper," Randy said finally. "How come?"
I don't know. Maybe I felt like playing hero."
"I wouldn't have. I would have let those kids burn to death."
"You might not have. You might have done the same thing."
Randy pulled out a cigarette and pressed in the car lighter. "I don't know. I don't know anything anymore. I would never have believed a greaser could pull something like that."
" 'Greaser' didn't have anything to do with it. My buddy over there wouldn't have done it. Maybe you would have done the same thing, maybe a friend of yours wouldn't have. It's the individual."

I got out of the car. "You would have saved those kids if you had been there," he said. "You'd have saved them the same as we did."