the weight of promises — fulfilled and unfulfilled — is a pretty big theme in one piece. laboon holds onto an unfulfilled promise from the rumbar pirates (or so we think at this point, having not yet been introduced to brook) and it clearly tears him up inside. sometimes bad things happen, and people can’t fulfill their promises no matter how much they want to. kuina fell down the stairs. the grand line proved too much for the rumbar pirates. and we don’t know the exact details of what happens during the void century, but we do know that joyboy made promises that for one reason or another he was unable to fulfill in his lifetime. so the people he made covenants with must wait for his spiritual successor to come in the future to take up what he left behind.
luffy is that spiritual successor, of course. he makes a promise here: he’ll be back one day, and he and laboon are going to fight each other again. he takes up the burden of the promise left unfulfilled (so we think) by the rumbar pirates. this happens in the very first arc of the paradise half of the grand line; and it’s neatly paralleled in the first arc of the new world, when we learn of joyboy’s promise to the people of fishman island and get a glimpse of the role luffy will play in that centuries-old promise.
