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.