One Piece begins with the execution of the Pirate King, Gol D. Roger. His death was intended to symbolize the power of the World Government, but had the opposite effect instead, conceiving the Golden Age of Pirates!
One Piece is full of mythology. What happened in the Void Century, anyway? What about the meaning behind the Poneglyphs? Gol D. Roger plays a massive role in this same mythology and is regarded more as a deity than as the fallible man he actually was. His first mate (Silvers Rayleigh, in episode 400 of the anime) provides us with a differing account of the Pirate King, a perspective not as much concerned with the legend as the man himself.
We learn that Gol D. Roger was dying of an untreatable disease at the time of his execution. He wasn’t caught in the prime of his life, but rather, just wanted to go out with a bang, in the words of Rayleigh, “In the last moment of his life, he (Gol D. Roger) turned his fading “flame of life” into a huge fire that enveloped the world.”
When Sanji asks Rayleigh what happened after they (Gol D. Roger’s crew) conquered the sea, his answer is blunt. They parted ways. The underlying sentiment here is that life goes on, and people come and go. This is said in the presence of the Straw Hats and it’s hard (and heart-breaking) to contemplate them ever separating, but it’s also affective in the way that it highlights fragility of the bonds that tie them (and, by extension, us to our friends) together, and how quickly those bonds can be severed.
Usopp excitedly butts in and tries to ask Rayleigh about Gol D. Roger’s famous treasure (the One Piece,) but Luffy jumps in before Rayleigh can give his answer, screaming that if he’s ever told such a thing, the Straw Hats may as well just break-up there and then, because their enthusiasm for adventure would be lost if they knew exactly where to look.
Usopp’s question might as well be “What is the meaning of life?” Just think, if some one provided you with an irrefutable, specific answer to that question, how would you feel? And how would you feel having finally fulfilled that meaning, too? From that point on, life would be pretty empty, surely? One wonders, how did Gol D. Roger feel at the end of his adventure?
If Luffy ever finds that treasure, One Piece is finished. Quite frankly, I hope he’s searching forever.