Basically, because Johnny (Sorry if there is a spoiler) had so many good morals to live by, and the fact that he barely had what someone would call parents, he learnt lots living with the Greasers. He learnt that people are just people and that wealth is the only thing that separates us. This helped Ponyboy to see the gang situation more clearly because what made up the SOCS was money. What made up the Greasers was a lack of money. And because Johnny was such a good friend and almost motherly figure to Ponyboy, he uncovered heaps of meaning behind his words when he said what he did in the letter.
