Ch. 45 page 5: “I decline. I’ve got a good reason for staying here.” — Sanji declines Luffy’s recruitment offer with a stated-but-unspecified reason. The chapter explicitly flags the reason as bigger than the surface-explanation Patty supplies later (page 15: ‘he wants to be head chef someday’).
The payoff
Ch. 56-58 (estimated) will reveal the Zeff backstory — Zeff sacrificed his leg (eating it, in fact) so the boy-Sanji could eat the food during a stranded shipwreck. Sanji owes Zeff a debt that ‘paying for his food’ cannot discharge; the year-of-chore-labor / decade-of-apprenticeship is a partial payment.
Why this matters
The withhold-the-reason technique is one of Oda’s most-used recruitment-pattern devices. The recruit’s ‘I have a reason’ line is the in-text flag that the recruit’s backstory chapter is forthcoming. Every Straw Hat has a reason the recruitment-resistance:
Zoro’s swords-and-promise (Ch. 5-6)
Nami’s village-debt (Ch. 14, full reveal Ch. 70+)
Usopp’s village-bond (Ch. 40)
Sanji’s Zeff-debt (Ch. 45 / Ch. 56-58)
Chopper’s Hiriluk-grief (Ch. 145-153)
Robin’s universal-betrayal (Ch. 218)
The Ch. 45 good reason line is the cleanest on-page instance of the device — the recruit declines, names the existence of a reason, and refuses to disclose it. The reader is primed by Ch. 45 to expect a Sanji backstory chapter.