Even though Holden and Pi come from two completely different cultural contexts, both of their stories focus on a similar goal: establishing meaning in one's life.
In both cases, this journey or quest of self-discovery is catalyzed by a tragic loss. In Holden's case, it is the loss of Allie, while in Pi's case, it is the loss of his entire family.
During this process, both of these characters question, analyze, and challenge society's expectations of them. While it is easy to recognize Holden's questioning of society because of his rebellious nature, one cannot forget that Pi also questions society's expectations by simultaneously practicing "opposing" religions. Furthermore, in the same way that Holden pushes his reader to view the world in a different way (through the adolescent identity and experience), Pi also does so by persuading the Japaneses investigators to see his story in a different way (through the importance of faith/belief in a “dry, yeastless” society).
Fundamentally, both of these protagonists seek to change their readers' perspective of the world.