Luffy had always wanted to be free— before he had even known of piracy, before his grandpa had taken a stern hand to his teachings; more than anything else in the world, Luffy wanted freedom. The rush of the wind as the beating of his heart rampaged wildly in his own chest, as if threatening to rip open and stomp off on its own. Freedom was Luffy's to have. Unfortunately for some people in his life, his freedom had come to him on a sunny day with the arrival of the Red–Hair Pirates, looking to wind down before they set off to the Grand Line once more.
From them, he learns that King of the Pirates is the most freest man of them all— doesn't remember if Shanks had been the one to tell him that, but he doesn't really care. He's already decided after all, Luffy is going to be a pirate, the Pirate King. The people in his circle take to this with varying degrees of support. Makino, bless her heart, for the most of it, seems generally supportive— in the kind way that an adult would support any child's dream, no matter how far–fetched. Garp, unfortunately, found himself so much closer to an early death.
His own grandson, one whom he was staking his lineage upon, his greatness— a pirate, all because of that darn Shanks. Cue some 'fists of love' and yet Luffy was still undeterred. The other children from Foosa had never taken to Luffy, apprehensive about his character and all his eccentrics. The only child he managed to befriend in Foosa was you. When he had told you that he was going to become a pirate after Shanks left, you looked at him like you were appraising him and then the look washed away to leave amusement behind.
You never seemed to believe him, amusing him only for the sake of amusing him— and if Luffy knew the word, he'd say you were patronising him. You were always a big know–it–all, but that's fine 'cause you were his first friend.
That's probably why Luffy couldn't find you that day he turned 16, ready to set sail. You were hiding from him, from the truth that you were always able to pass off as a joke— even after he met a wild Sabo and Ace who only encouraged his dream. Luffy spends half the afternoon trying to find you to say goodbye, only to find you when he starts to sigh along the docks of Foosa.
Your back to him, gaze set out upon the sea. Luffy imagines the angry look on your face— yikes— and winces before he says, "I'm saying goodbye," he sounds almost mad himself, "you can't run, so listen to me." He follows it up with, "coward." Just for good measure.