Ponyboy tells us in the novel "The Outsiders," that he is a member of the gang called "Greasers" These guys are outsiders because they don't fit in with the group called the Socs. The Greasers are from broken homes, poor economic situations and little education. The Socs have the best of everything, which makes them more popular. As Cherry tells Ponyboy,
"You greasers have a different set of values. You're more emotional. We're sophisticated—cool to the point of not feeling anything ... Rat race is a perfect name for [our life]." This leads Pony to wonder if perhaps it is just natural for the two classes to be separate and unequal—a fact that haunts Johnny's decision to turn himself in. He knows that the courts stereotype all greasers as juvenile delinquents."
Pony is also somewhat of an "outsider" in his own group. He is younger than most of the other boys. He is still in school and he is really pretty bright. This causes a feeling of isolation for Ponyboy sometimes and he really doesn't understand the distance he feels.
Ponyboy is considered an outcast throughout society because he occupies a lower social class and is a member of the delinquent gang, the Greasers. Pony's status as a Greaser makes him seem like a juvenile criminal who is only interested in terrorizing the neighborhood and fighting Socs. The Greasers are viewed with contempt throughout their community and have a bad reputation as ignorant teenagers who enjoy breaking the law. However, Ponyboy is not like the other members of the Greaser gang. He is a sensitive, intelligent, quiet individual who does not enjoy harming others. Pony's unique personality and young age also make him an outcast in the Greaser gang. Unlike the other members of his gang, Ponyboy has personal conversations with Soc members and begins to understand their perspective on life. Pony's negative reputation throughout society and his unique personality amongst his group are the reasons he is considered an outcast throughout the novel.
Ponyboy is an outsider because he doesn't really fit in to either group in town. While he is technically a Greaser, his intelligence and sensitivity mark him as an outsider in that group. His association with the Greasers, and his economic and family circumstances make him an outsider at school with the Socs.
Because he is a greaser and greasers are considered to be outsiders in this book.