The core of the plot is naturally unbelievable. Truman Burbank is a seemingly common guy who isn't common at all. His life is actually one sensational 24/7 television show in which everyone but him is a participating actor. The plot evolves as Truman's psyche devolves - as he realizes that something is fundamentally wrong with his perfect yet shallow life. The whole movie builds brilliantly to a culminating third act - an emotionally moving progression that always gets me.
I'm not quite sure why this film resonates so strongly with me, why I always watch it when it is on television. It could be the predictive nature of the plot - that it strongly foreshadowed the rise of the absurd phenomenon that is reality television. It may be the strong performances, my respect for Jim Carrey, or the overall quality of the production. Or it could stem from my core need to find meaningful context in every-day reality.