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Sanji would do anything for Luffy.
Luffy gave him back his dream, his ocean, his All Blue, after Sanji had abandoned it. He reminded him that was out there somewhere, waiting for him to find it, and gave him the greatest opportunity imaginable for him to go chase it. In return, Sanji decided he would do whatever it took to make him Pirate King.
He'd left the only home he'd every loved to follow a boy with a straw hat and a dream, and he never had any regrets.
He'd taken on an avalanche for Luffy, stayed in his own personal hell for two years because Luffy asked him to, gotten stronger and became a better chef so he could be someone Luffy could rely on. He followed him across deserts and into the sky, stormed government strongholds with him, would follow him to hell and back in a heartbeat. Luffy was his Captain and Sanji would do whatever he needed him to.
He left Dressrosa and took half the crew to Zou because Luffy asked him to. He didn't want to, it felt too much like running away and leaving his Captain and friends to fend for themselves, but Luffy wanted him to, so he did. He kept the crew safe and together until the others were able to meet up with them.
Unfortunately the Big Mom Pirates came for them first, and Sanji did what he thought was best. Save the rest, let Capone take him, resolve this thing with the Vinsmokes once and for all, and make his way back to the crew when it was all said and done. He left Luffy a note so he wouldn't chase after him and resolved to finish this matter without the others getting dragged in and hurt for his sake.
Then things got complicated.
The Vinsmokes had finally found a use for him, and they weren't just going to let him waltz away again. They shackled his hands, threatened Zeff, the man who saved his life, the man who was more of a father to him than his actual father. They'd stolen his freedom and forced him back into this family, into this marriage to a woman who he didn't even know. The Vinsmokes had taken almost everything he cared about away from him in only a few short days and Sanji couldn't even stop it from happening.
And when he saw Luffy racing towards him, relived smile plastered across his face, he knew they would take him, take them all away from him, too. Sanji couldn't, wouldn't let that happen, not if he could still do something about it.
"Leave, you filthy, lowly sea slugs."
It hurt to say, it hurt to think, hurt even more when he saw the confusion and heartbreak flit across Luffy's face until he eventually settled on anger and rage. But he could take the anger and the hurt if it prevented Luffy and the others from becoming another threat for the Vinsmokes to blackmail him with, could let his friends keep sailing to the ends of the sea. If Sanji couldn't be free he would make damn sure Luffy was.
"I'll be the one to take all this trash back to where they belong."
Sanji would do anything for Luffy, even if he had to break his heart.