Monkey D. Luffy is actually Gol D. Roger, and he's a time traveler.
Remember when Roger's crew disappeared in the Grand Line? That's because they all traveled somewhen else. They did reemerge, but most of the crew lost their memories of when they traveled...except of course Roger, who was transported to an 8-year-old who eventually ate the Gum-Gum Fruit. Being in this new body, Luffy, who still had his memories from the previous life, was incredibly optimistic about joining Shanks' crew. After all, Shanks was his protege when he was in his crew. That's why he gave him the straw hat.
When Shanks made the promise to "Luffy" and gave him his hat, Roger couldn't bear to pressure him more. The fact that he gave him his hat touched him so much that he couldn't do anything except cry. Therefore, Roger decided to start over and become a new pirate king to follow his own legacy. This is also seen when he visits his death pedestal in Loguetown.
Roger wasn't sure why he was in this life, but he knew for a fact that this was the future. He couldn't keep his temptations away, so he visited where he died to reminiscence on the entire situation. When he was almost executed by Buggy, he couldn't help but smile. It was amusing to think that you are going to die in the same place twice. He was certain he was going to return to his older life eventually anyways, since as history says, the Roger Pirates reappeared after some time of vanishing.
Fast-forward to years later, and the Straw Hat Pirates are nearing Raftel Island. However, something happens. All of a sudden, everyone disappears! This is just like what happened in the legend. It's unknown what happened to the other crew members, but for Luffy, he was shocked to find that he returned to his original time. He was Roger all over again! It was hard for himself to believe; especially the fact that he's the only one who knows everyone disappeared. Still, he was amused and satisfied. He decided to turn himself in, pretending to have an illness, and announce his treasure to the world so he can start that fun time all over again! He smiles in his death, knowing the legend he has started, and the lives he has lived. He was happy.
Why do you think Whitebeard was so generous to Luffy during the war? Or why he refused to back down even in his death? Because after the whole thing, Roger told him about his adventures. Whitebeard was proud, so he vowed himself to help Roger even in his second life. This also explains why Luffy reminded Rayleigh (who didn't know about the life) of Roger when they were with each other. Also, Luffy has always been laid back with Buggy. He knows how cowardly ambitious he is, but he loves him.
Now one question remains: If he knew everything about his previous life, why did "Luffy" follow his own success? Why did he want to know what the One Piece is if he already knew it? This is likely because he didn't know about it. All he knew was that when he returns to his original body, he will find a legendary treasure. However, since Luffy disappeared, he never got a chance to find it. That is, until, when he returned to his original body. It was then that he realized what his treasure is. He learned the most important thing about an adventure is not the destination, but the journey. He's had a chance to conquer the world twice, and live an adventure unlike any other. His life has a young rookie adventurer shows how much will he has. It was his Will of D. His life as Monkey D. Luffy...was just one piece of his life.
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