So many fans are disappointed with Sanji’s portrayal in Whole Cake Island. Specifically, many believe that Sanji’s loyalty to Luffy was shown to be very feeble. This is because Sanji prompted a fight with Luffy in the hopes that Luffy would abandon him (which, fans believe, is something Sanji should have known Luffy would never do), and because Sanji was reluctant to seek Luffy’s aid in stopping the wedding and saving his family.
These fans believe that Sanji underestimated both Luffy’s loyalty to him and Luffy’s strength, which is uncharacteristic and does not befit Sanji.
Of course, this is a misconception. Sanji did not lack faith in Luffy’s strength. Sanji was reluctant to look to Luffy for aid because Sanji is an inherently selfless person. It is in Sanji’s nature to shoulder a burden alone; Sanji is not the kind of man who enjoys burdening others with his own problems. He was worried that his selfish desire to stop the wedding would bring serious inconvenience to Luffy and the crew, so he chose not to involve Luffy (until, of course, Luffy insisted on being involved).
We must also consider Sanji’s tendency to keep his emotions hidden. Sanji does not like revealing weakness; this is a characteristic that he developed as a result of his upbringing, for he was abused by his father and siblings for showing weakness.
To be precise, Sanji doesn’t like to show emotion in general. He behaves in a stoic manner but is very passionate within, which is something that is apparent only through Sanji’s intense anger. It is only when Sanji is alone that his vulnerabilities and passions are exposed.
Out of habit, he wouldn’t reveal to Luffy the problem he was having, and wouldn’t expose to Luffy his weaknesses. That’s why his problem was solved when Luffy insisted that Sanji reveal his true feelings in chapter 856; Sanji finally found the courage to open-up to someone, and was able to divulge his feelings of despair.
Lastly, I wanted to discuss the fight between Sanji and Luffy in chapter 844. Inly, Sanji understood that Luffy would never turn his back on him, regardless of how behaved. Nami fell for his facade, and every other crew member would have fallen for it as well. However, Luffy is the only crew member who understands Sanji thoroughly, and he dismissed Sanji’s lies easily. Sanji didn’t truly believe that Luffy would abandon him; Sanji only wished that Luffy would do so. There’s a huge distinction.
*Theory by Navigator BlazeItSword