Monkey D. Luffy, the protagonist of One Piece, is often seen as a character who spurs on the rest of the Straw Hat crew to grow and change rather than a hero who grows and changes much himself. While this analysis is fair to an extent, it would be wrong to say Luffy hasn't learned anything at all in his journey across the Grand Line.
The following five scenes from throughout One Piece have greatly affected Monkey D. Luffy, providing him with a strong character arc over the course of the series.
Luffy Receiving Shanks' Straw Hat
The most pivotal moment in Luffy's life happens when he's just a little kid. The boy challenges Shanks by not only saying he'd become an even better pirate than him one day but that he'll even become the King of the Pirates. Shanks doesn't laugh at this proclamation by the young rubber boy like he did when he asked him to take him out to sea. Instead, Shanks smiles, because, as he would later say to Rayleigh, Luffy "said the same thing that Captain Roger did."
Responding in kind, Shanks gently places his coveted Straw Hat on Luffy's head, asking him to one day return it when he does become a great pirate. This hat means a lot to him, so he asks Luffy to take good care of it. This moment changed Luffy's life forever, spurring him to go out to sea in the first place.
Luffy Fighting Usopp for Ownership of the Going Merry
The Water 7 arc forces Luffy to really take his captain's duty more seriously. The Galley-La company tells him and Nami that the Merry can't be repaired. At first, Luffy responds as stubbornly as you'd expect, not wanting to abandon the ship that got him and his Nakama this far, but when Iceberg then questions Luffy's ability as a captain to do something so irresponsible, Luffy is unusually shaken up.
Luffy eventually realizes he must do what's best for the Straw Hat Pirates as a whole, so he makes the decision to ahead with a new ship. Usopp finds Luffy's disrespect towards their previous ship inexcusable, leading to a duel between the sniper and his former captain. Luffy beats Usopp, but this is not a celebratory victory -- he's hurt his former friend. It's here Luffy truly realizes that a captain's job is not an easy one, and not everyone will agree with your difficult decisions.
The Straw Hats' Separation at Sabaody Archipelago
It's no secret that failure is a better teacher than success, and Luffy's face-off against the Pacafistas and Admiral Borsalino in Sabaody is his biggest brush with failure. The Straw Hats are outmatched, and Luffy has to make another difficult choice by telling the crew to retreat and scatter, with the hope they'll reconvene soon. Unfortunately, they don't even make it that far as Bartholemew Kuma makes a sudden appearance.
Without any warning or apparent reason, Kuma begins popping the Straw Hat Pirates away with nothing left behind of them. It's upsetting to watch Luffy run and flail around trying to save his Nakama from a faith he doesn't understand. In the end, he lays in the grass, ashamed of how weak he is. This failure leads him to the very burdensome verdict to have the crew stay apart for the next two years, so they can better equipped to handle the New World.
Luffy Meeting Jimbei
Luffy's first meetings with all his crew hold personal importance, but meeting Jimbei is something different. Luffy meets Jimbei in Impel Down at a time when his allies are few and far between. However, he makes allies from Newkama and unexpectedly survives the various perils of Impel Down. Seeing Luffy survives this much makes Jimbei realize that Luffy could actually save Ace.
Over time, their relationship develops to that Luffy asks the former Warlord to join his crew in Fishman Island. Jimbei, unfortunately, turns the offer down as he has other responsibilities that need to be taken care of first. During the Wano arc, however, he commits himself entirely to Luffy's cause. In a sense, Luffy's relationship with Jimbei teaches the rubber man not only the value of unlikely allies but also that good things will come to those who wait.
Luffy Losing Ace
Since getting separated from his crew, Luffy has been facing roadblock after roadblock trying to save his brother. Finally, he manages to save Ace from his execution. Despite this, destiny had alternative ideas, as Akainu tore a hole through Ace while protecting Luffy. Misery washes over Luffy like a violent tsunami as he watches his brother die in his arms.
This moment changes Luffy -- for the first time, he wavered on his dream. For the first time, a boy once so unshakable in his resolve surrenders. What chance did he have of becoming King of the Pirates if he couldn't even save his brother? Fortunately, Jimbei's there to give him the strength he needs to keep fighting, reminding him of his crew and there would be good times to come. Luffy did eventually move on from his brother's death, but he still keeps him close to his heart.