Luffy's dream is to become the Pirate King in One Piece, yet it all started back in his early childhood.
Even the most casual fans know that Luffy wants to follow in the footsteps of Gold D. Roger. Right before the latter's execution, the Pirate King said there was a great treasure out there for anybody to find. This is what started the Golden Age of Piracy in the One Piece world.
However, Roger is not the man that inspired Luffy. Rather, it was a former apprentice by the name of Shanks. Both of these great men have worn the signature Straw Hat, which has now been inherited by Luffy. To become the Pirate King is to share their love of freedom.
Why Luffy wants to become the Pirate King in One Piece
Pirate Kings have to be extremely powerful, with vast amounts of political influence in the world. They also need strong allies and an undying resolve to live. Luffy's journey to find the One Piece begins in Dawn Island.
Luffy made a promise to Shanks
More than anybody else in the One Piece series, Shanks is directly responsible for Luffy's ultimate goal. Back in the East Blue Saga, the Red Hair Pirates used to regularly visit Foosha Village. This is where Luffy grew up as a child.
Luffy always looked up to Shanks for his adventurous spirit and carefree attitude. He wanted to join the Red Hair Pirates, but was considered too young. Luffy even gave himself his famous scar to prove his toughness, but to no avail.
However, Shanks always believed in Luffy's potential to grow. At one point, he even saves the boy's life from a dangerous Sea King. Shanks sacrificed his entire left arm just for Luffy, which says a lot about their relationship in One Piece.
Before the Red Hair Pirates left Foosha Village, Shanks gave Luffy his trademark Straw Hat. The latter promised that one day he will become a great pirate, surpassing even Shanks himself.
Luffy owes a debt he can never truly repay to the Red Hair Pirates. If it wasn't for them, the Straw Hat would've never eaten the Gomu Gomu no Mi (albeit by accident). Luffy's diverse fighting style is the result of his relationship with Shanks.
What it really means to be a Pirate King
Luffy explains his ambitions at the end of the Dressrosa arc, right after Doflamingo's defeat. With a Grand Fleet at his disposal, the Straw Hat delivers a very empowering message.
He doesn't want to become a Pirate King just for the mere title. Instead, Luffy wants to be a free person, just as Shanks taught him earlier. The captain of the Straw Hats also values adventure, including the freedom to make choices.
Roger's first mate Rayleigh understood this back in Saboady Archipelago. Before he could reveal the true nature of One Piece (per Usopp's request), Luffy angrily declined the offer. He wants to find out the truth by his own merits. Pirate Kings must figure it out for themselves.
Note: This article reflects the writer's personal views.