Ponyboy learns that despite the obvious differences between the Socs and the Greasers, he has the scope to be whatever he chooses. He learns this through his relationships with those on both sides of the battle: his brothers, the enigmatic Dallas Winston, the brave Cherry Valance and his tragic friend Johnny. It is in Johnny's letter where Ponyboy finally understands the significance of the line from the Frost poem, 'stay gold'. Ponyboy writes the account of the events which have affected him so much, and which have shown him the value in everyone around him.
Maybe the two different worlds we lived in weren't so different. We saw the same sunset.