Here's why:
* Opposing Personalities: Ponyboy is sensitive, idealistic, and values being a "greaser." Darry is tough, pragmatic, and burdened by responsibility. Their contrasting personalities highlight the pressures and choices facing young men in their situation.
* Different Paths: Ponyboy is struggling to find his place and his identity. Darry has already made choices and sacrificed his dreams to take care of his family. This contrast emphasizes the challenges of growing up and making decisions.
* Love for Family: Both Darry and Ponyboy love their family deeply, but they express it differently. Darry shows his love through tough love and discipline, while Ponyboy is more emotionally demonstrative. This contrast shows that love can manifest in various ways.
* Themes of Class and Society: The conflict between the Greasers and the Socs (wealthy kids) is a central theme. Darry's working-class background and his struggle to make ends meet directly oppose the privilege of the Socs, which Ponyboy, despite his poverty, also desires.
By portraying Darry as a foil to Ponyboy, Hinton allows readers to see the complexities of the Greaser world, the internal conflicts within the Curtis family, and the influence of social class on their lives.