It started with a simple question; 'why do you all call him pops?'
For Marco, it was easy. He was outcasted as a child and Whitebeard found him, taking him in and bringing him back home to Sphinx Island. The boy quickly grew to call him father and then pops. Of course, the old man welcomed him as the first of his crew without hesitation.
For Thatch, it was a bit more complicated. He had studied under several proud chefs and worked hard to earn his position at the resturant. The island ended up being raided by pirates and he fought back, defending and helping whoever he could. By the time Whitebeard showed up, the town had been ravaged but Thatch continued to fight back. Whitebeard liberated the town and offered Thatch a position as head chef. Of course, Thatch happily took it and began sailing.
Izo had taken time to join. He was living quite comfortably in Wano, teaching young ladies proper manners and how to play a variety of instruments. He was paid quite well and had nothing to complain about. Whitebeard's crew was growing quite well at that point but they had no proper musicians to help them celebrate. Izo only agreed to join under the circumstance he was given his own private quarters to practice and prepare.
Jozu was originally a slave, carrying around rich men and doing any and all heavy lifting. They found him on Sabaody and he asked to join in secrecy, swearing his life to theirs in exchange for freedom. They freed him and gave him a new home and new life. It took him time to adjust, but he did easily enough and called Whitebeard pops before an entire year.
Vista asked to join, preparing to fight Whitebeard himself to prove his worth. Whitebeard humored him and they scuffled for a brief fight, Whitebeard easily beating him but still very impressed. Vista happily joined, training others and expanding his own reputation.
Ace never had anyone to call 'father' before. He had Dadan and the bandits, but they never referred to each other as family. He really only had his brothers for family and even then it wasn't much and still no father.
On Whitebeard's crew, though, everyone was family. Everyone had each other's back and was more than happy to help each other. Even Whitebeard's allies would offer the shirts off their backs if the old man asked for it.
Everyone called him their father; everyone called him pops. Each for their own reason, but they all looked up to him. And he welcomed them with open arms, he accepted everyone regardless of status or strength. Even the nurses were his daughters and they were banned from battle.
It still didn't quite make sense to him, but he kinda liked the feeling. He'd never had an actual family before and to be welcomed with open arms... he really liked that feeling. To belong to a family, that would feel so liberating.
Ace took a deep breath and stood up, curling his fingers by his sides. Yes, he would proudly accept the mark and accept that he would be part of a family. He would belong to a family after years of being without one. And he would gladly help his family however they needed.