Ninth Son of Great Qin: "In a sense, Whitebeard's reputation in his world has now surpassed even that of One Piece Roger."
Ninth Son of Great Qin: "With Whitebeard spurring him on, it is only natural that he will grow faster."
Even if One Piece was a real world, as the protagonist of the Pirate World and the Joy Boy of this generation, Luffy's future had already been determined. He was destined to become the Pirate King.
Although Whitebeard had no desire to become the Pirate King, he was still a mountain Luffy had to overcome if he wanted that title.
Pirates revered strength. While whether one could become the Pirate King was not directly related to that, and depended more on whether one could find One Piece, without sufficient strength, one could not change this era.
"Joy Boy" was someone destined to change the world. He would bring laughter to the world, happiness and joy to people. Without strength, he could never accomplish such a thing.
When Kaido defeated Luffy for the second time, he looked at Luffy falling into the sea and sighed, "So you couldn't become Joy Boy either."
That word, "either," meant that before Luffy, there had also been people Kaido believed possessed the potential to become "Joy Boy." Yet for various reasons, they had all ultimately failed.
In Luffy, he saw the shadows of many people: Roger, Whitebeard, Rocks, Shanks, Oden. These were all people he had once believed might become "Joy Boy," and the one thing they all had in common was immense strength.
They might not necessarily have been able to change the world, but they were all powerful figures standing at the pinnacle of the seas.
Kaido had noticed Luffy partly because of his strength. But after Luffy's defeat, Kaido's words also