the tragic death of his parents in an automobile accident. Such a devastating bereavement at such a young age would cause many people to retreat into their shell. Ponyboy's developed a sense of empathy for others and their pain due to his parents' death, and this sets him apart from the other Greasers.

As for Ponyboy's being quiet, well that's mainly because he knows he's only really accepted as one of the gang on account of his being Darry's brother. Ponyboy's sensitivity, as well as his relatively small size, doesn't exactly make him the ideal gang member. Under the circumstances, Ponyboy figures it's best to keep his mouth shut and let others do the talking.

There are several reasons why Ponyboy is quiet and sensitive. Ponyboy himself puts it quite simply: “I’m different that way.” Even though he was raised in the same environment as Sodapop and Darry, he prefers books and movies to drag-racing and rodeos. He likes quiet experiences that allow him to observe and reflect. Ultimately, S.E. Hinton characterizes Ponyboy this way to show how not all members of a group are alike; even within a close-knit “family,” there will be a variety of character types. Just because Ponyboy is a greaser doesn't mean that he's loud and brash. 

Ponyboy also might be quiet and sensitive because of the pressures of his home and school environments. Ponyboy is still mourning the loss of his parents, and even though he doesn’t mention it much in the novel, we can assume that this tragedy deeply affected him. He’s under pressure at school, where he learns and studies in advanced classes alongside the Greasers' enemies, the Socs. He’s also under extreme pressure at home, especially when Darry holds him to a rigidly high standard.