Disclaimer: I do not own One Piece.
Sanji wasn't stupid, despite what others may think due to his flirtatious and overly amorous behavior (it was perfectly fine to admire beauty, and women were beautiful. How could one not be enamored?)
He had noticed the shift.
There had been hints before arriving at this level, of course. Hints from the start. From the very beginning, after all, none of them could resist him. When he smiled that full smile that took up half of his face, and asked them to join him, none of them could refuse. How could they? He saved them, each and every one of them, provided them with a home and a family to return to. He had shown them that anything was possible, no matter how unlikely it may seem. He called them nakama.
Of course, they all had other reasons for joining, at first.
Zoro wanted to become the world's greatest swordsman.
Nami wanted to draw a map of the entire world.
Usopp wanted to become a brave warrior of the sea.
Chopper wanted to become a doctor that can cure any disease.
Robin wanted to find the True History.
Franky wanted to create and travel with a dream ship.
Brooke wanted to return to Laboon.
Sanji wanted to find All Blue.
And so they sailed. The hints only became more obvious as time passed. How they followed him unquestioningly into increasingly perilous situations, nearly losing their lives, along with any hope of achieving their dreams, several times. How, while most would be thankful to be away from the danger, they wished they were present to storm the gates of Hell itself with him, and were jealous of the ones who had the opportunity. How, when they heard his brother was murdered before his eyes, they were overwhelmed by a white-hot fury and each contemplated how to kill Akainu slowly and painfully.
How, in the 2 years they didn't see each other, their main concern was getting stronger for him. To not disappoint him. To support him. To be worthy of him.
They got mad when someone dismissed him. They got brutal when someone insulted him. They got murderous when someone dared to hurt him.
They did not take the implication that they were disloyal well.
Brooke was offered stardom. If he abandoned the Straw Hats, they said, if he abandoned his captain, then they would make him into a star the likes of which had never been seen before. They promised money, fame, women who were willing to flash their panties. Brooke yo-ho-ho-hoed. They had nightmares of laughing skeletons and demon kings for the rest of their lives.
Franky was offered Dr. Vegapunk's research notes if he told the marines where his captain was planning on heading next. Only half of them made it back to the base. Those who did developed an acute fear of Cola.
The World Government promised Robin that they would pardon all of her crimes if she handed over her captain's head. They even promised to give her a clue as to the whereabouts of the Rio Poneglypgh. They found the head of the messenger on their doorstep the next day.
Chopper was invited to a pirate crew of doctors. They asked him to poison his captain, and in compensation they would accept him into their crew and help him conduct research to find the cures for various diseases. From that day on, the members of the doctor crew would swear that the Straw Hats harbored a pet monster that mauled most of its enemies and crushed the rest.
Having heard that there was conflict in the past (all the world followed the Straw Hat crew's actions closely, partly incredulous, partly fearful, mostly awed), an enemy pirate crew asked Usopp to provide information on his captain. Usopp, who had been cowering in fear at the sight of the entire crew of tough-looking men surrounding him, stopped trembling once he heard the crew's proposition. He readied his slingshot. The pirates were blinded for life.
There was a noble who had heard of Nami's deceptive ways and told her he'd pay her a small fortune if she helped him capture her captain. Nami smiled prettily. The next day, the noble disappeared, never to be heard from again. His fortune, however, mysteriously appeared aboard the Sunny the same day.
Zoro was offered a place in one of the strongest pirate crews on the Grand Line. The captain proclaimed he'd find the shortest way for him to become the world's strongest swordsman, and promised a series of sparring partners available in his 500-man crew. Zoro laughed, and proceeded to give the presumptuous captain a large scar for his audacity. The scar possessed an eerie resemblance to a skull wearing a straw hat.
Thinking about these instances, Sanji couldn't help but think they each deserved it. Implying that the Straw Hats were disloyal was the gravest insult they could give. It required punishment.
Sanji sighed. The shift really was obvious, now that he thought about it. Somehow, along the way, without their noticing, their main objectives changed. He laughed. It seemed so long ago that his priority was finding All Blue. He still wanted to, of course. It was his dream, after all. But nothing compared to the fervent desire to continue standing proudly by him.
They still followed their dreams- that had not changed. Their motives just shifted. No longer did they do it solely for themselves. Now, they pursued it with the acute knowledge that he deserved only the best as his crew. They did it for him. It would always be for him.
He deserved the world's greatest swordsman, a navigator who mapped the world, a brave warrior of the sea, a cook who found the legendary ocean, a doctor with cures for all the diseases in the world, an archaeologist with knowledge of the True History, a shipwright who built and rode the greatest ship, a musician who fulfilled his promise.
They would become those things to stand by him.
After all, Luffy deserved only the best.
He was the future Pirate King.