"Dead Poets Society" is considered a masterpiece due to its powerful exploration of the teenage experience, its poignant portrayal of the teacher-student relationship, its timeless themes of seizing the day ("carpe diem"), the impact of a passionate teacher like Mr. Keating on young minds, and its ability to provoke deep reflection on the importance of individuality and creativity, all delivered with a compelling narrative and exceptional performances, particularly by Robin Williams as Mr. Keating.