Ponyboy doesn't get along with his oldest brother Darrell. He and Darry argue about rules and priorities. Darry wants Ponyboy to think more concretely, to plan all his days, to get good grades but to be athletic and active. Ponyboy likes to daydream and think about philosophical matters, and reads and goes to movies more than Darry would like.
Through all this arguing, Sodapop is the shoulder that Ponyboy cries on. He likes Sodapop because Soda listens to his ideas, shows interest in what he reads, etc.. Soda is sympathetic about Darry and helps to calm Darry down, often taking Ponyboy's side. Soda is light-hearted and cheerful, and Pony enjoys his spirit.
However, what Ponyboy comes to realize is that while Soda is always there for him, he hasn't been there for Soda. He doesn't ask Soda about his life, about Sandy, about how he is feeling. He doesn't think about how the fighting between him and Darry might be hard for Soda, and that he is putting Soda in the middle:
I don't know, man. It's just like sometimes I have to get out. It's like I'm the middle man in a tug-of-war or something between you guys. I don't know, I can't take sides.
Ponyboy's realization about Sodapop shows that he is maturing. He is starting to think about the situations and the feelings of people around him, to accept that "things are rough all over" and to be considerate of other people's challenges.