Luffy is the one piece.

In the One Piece universe, pirates symbolize freedom. Therefore, Luffy wanting to become ‘king of the pirates’ actually means he wants to become ‘king of freedom’.

In the Bible, it says that Jesus died for our sins. In a similar sense, Luffy will sacrifice himself to liberate everyone else from corruption (you know how the world government is corrupt in the One Piece universe). In other words, everyone will become free.

Also, there is this theory that Shankz knew that the devil fruit Luffy ate wasn’t the gum gum fruit, and he has actually planned for Luffy to eat the fruit. If that theory is true, then Luffy becoming king of the pirates has always been his destiny. It has been pre-determined by outside forces, which is similar to the story of Harry Potter.