Since the beginning of One Piece, Sanji has stood out as one of the strongest fighters of the crew alongside Luffy and Zoro. Even though he's the ship's chef, his strength is great enough that he rounds out the crew's Monster Trio. Having found a new family among the Straw Hats, he has routinely fought to protect them against their ever-growing list of increasingly powerful adversaries.
On multiple occasions, most notably during the Enies Lobby arc and two-year time skip, Sanji pushed himself to get even stronger, acquiring Diable Jambe and Haki respectively. During the recent raid on Onigashima, Sanji has undergone another upgrade, this time related to the Germa 66 enhancements that made his siblings so powerful -- so why is his most recent upgrade so upsetting to him?
Growing up as a prince of one of the most technologically advanced kingdoms in the world, Vinsmoke Sanji's life should have been a relatively easy and joyous one. Unfortunately, his father, Vinsmoke Judge, had other plans for his children. Performing experiments on them while they were still in the womb, Sanji and his other brothers were to be born with incredible physical abilities and enhancements, as well as be without the weaknesses of sympathy and empathy.
It was only with the sacrifice of his mother, Vinsmoke Sora, that Sanji was spared this fate. By taking a drug meant to undo Judge's experiments, she hoped it would save her children from growing up as the sadistic monsters their father hoped they'd be. Sadly, only Sanji was spared this fate, with his father and brothers using it as justification for their constant abuse toward him.
Over the course of his childhood, Sanji was met with nothing but cruelty and pain from his family. His father hated him for being weak, while his brothers found fun in exploiting that fact by routinely showing how much stronger they were than him. His mother was the only one who showed him any love or warmth, but tragically, the drug she took to counter the genetic enhancements on Sanji and his brothers also caused her death.
Not long after his mother's death, Sanji was thrown in a dungeon to keep him out of his father's sight, which led to even more abuse from his brothers. It wasn't until the Germa Kingdom invaded another nation that Sanji found his opening to escape. However, in the process, he ran into his father, who didn't just aid him in his escape but encouraged it. Seeing only an embarrassment and a failure, Judge's only request to his son was that he never reveal his origins, as it would supposedly only lead to disgracing the Vinsmoke family name.
The next time Sanji saw his family, they only desired his return in order to use him as a pawn. Hoping to ally themselves with the Big Mom Pirates, they wanted Sanji to marry one of her daughters. To get him to comply, they blackmailed him by threatening the lives of the Straw Hats and the Baratie crew that raised him after he escaped. This ordeal ended with Big Mom betraying the Germa, which forced Sanji to save them.
With all this being said, it would be easy to understand Sanji's feelings toward his family and heritage. Aside from his sister Reiju, Sanji hates his family to the core. Even though he chose to save their lives during the Whole Cake Island arc, Sanji only did so out of the kindness in his heart, not because he forgave or loved his family. Even the Raid Suit that was gifted to him afterward is something he's only used twice so far -- once to see what it was capable of, and another time so he could peep on bathing women.
While Sanji isn't happy about his Germa enhancements awakening, it's important to note that he does want to get stronger. Sanji has more often than not been placed in a lower tier than Luffy and Zoro, even though the three together make up the Monster Trio. While he is meant to be the crew's chef first and foremost, he is also keenly aware of his role as one of the wings of the next Pirate King, and as such, he knows he needs to be strong enough to support Luffy.
Unfortunately for Sanji, his most recent power-up goes against everything stands for. He hates the Germa and the Vinsmokes and wants nothing to do with them. Worse yet, awakening his Germa 66 powers through the use of the Raid Suit makes his mother's sacrifice all but pointless in his eyes.
While it has yet to be seen exactly what enhancements he has and how far they go (such as if they will neuter his ability to care about others), it's clear that Sanji is afraid of ending up like his family. For someone as kind as he is, it's hard for him to see his Vinsmoke heritage as anything less than evil or his new powers turning him into nothing more than a monster.
