''Orange Town Arc'' episode 4

It starts with episode 4, which is a special one for going backward to revealing Luffy's past. The appearence of  the pirate ''Red-Haired Shanks'' and his great influence on Luffy's dream.
Shanks is known as a great pirate who has a vast elite crew and a gigantic ship. the meeting was in Luffy's hometown Fuusha village, they were drinking and partying at Makino's bar, when a group of mountain bandits entered in a rough manner and had a little fight with Shanks, In that moment Luffy was pretty angry at Shanks for not fighting back and just laughing off the problem, but Shanks said: ''they only spilled some sake, it's nothing to get angry over.'' now that was a pretty calm and cool sentence, this shows us that he is not the type of person that goes after or gets angry over petty things.
then out of tantrum Luffy began eating the fruit he found in a box nest to him, that was one of the things that totally changed his way of life. It was one of the devil fruits the Gomu Gomu no mi, by eating it he became a rubber man who is able to extend and shrink his limb and other body parts in exchange for losing the ability to swim.
Well the possibility of gaining a special ability is truly awesome but becoming unable to swim is really so bothersome, especially in the world of one piece which is based on sailing being a good swimmer is truly essential.
Afterwards, Luffy went to the bandits to insist on them to apologize for what they did earlier in the bar, instead he got caught by their boss and dragged and thrown in the sea.
While struggling with all his might not to drown the giant sea king living in that area appeared and killed the bandit. And as it was heading toward Luffy, Shanks appeared and saved him from the sea king by just glaring at it. But unfortunatelly in order to save Luffy Shanks lost his left hand, and while Luffy was crying for Shank's hand he said: ''just one arm is a cheap thing to pay, I'm glad you're safe.'' with that shanks became Luffy's Life savior
At the moment of Shanks' departure Luffy said: ''I won't ask you to take me with you anymore, I've decided to be a pirate myself ... no matter what I'll become the pirate king.'' in that moment Shanks  took off his precious straw hat and gave it to Luffy as a sign of that promise
In here we can see that the trigger of Luffy's departure was a promise made with a dear person, this shows us that the series of one piece is revolving around how people are fghting and pushing forward to keep and fulfill their promises.