This scene with Sanji is one of his most important moments in the series, and in my opinion also one of the most difficult passages in the manga to interpret, because to completely understand it you have to look at the manga holistically rather than this scene by itself.
The big question that needs answered is why does Oda let Zoro "win" here by having him be the sacrifice instead of Sanji. Both are equally willing and both are identified much later on as the Wings of the Pirate King, implying that they have similar importance (although vastly different roles) within the Straw Hat crew.
If we go back to Sanji's introduction on the Baratie, his big flaw was that he lacked the "spear of spirit" to pursue his dream. Since he's been a boy he's wanted to find the All Blue, but even when he had the opportunity to go after that dream he chose to stay on the Baratie out of a feeling of obligation to Zeff. Sanji put the continued existence of the restaurant over his own life, something Luffy rightfully called him out for at the time, and even at the end of the arc had to be pushed away by Zeff and the other chefs before he finally set sail for good.
On Drum, Sanji once again almost died protecting Nami and Luffy during the avalanche, resulting in a broken back that required surgury from Dr. Kureha. Luffy again calls him out (note the English translation here isn't entirely accurate, see here for a breakdown), and with his power there's a good chance Luffy could have gotten them all out of trouble without all the dramatics by Sanji.
Something similar happens on Skypiea, when Sanji puts himself in the way of Enel so that Usopp and Nami can be saved. This case is perhaps more justifiable given the extreme situation they were in, but nonetheless he was still quick to throw his life away.
Then on Enies Lobby Nami--while not criticizing his chivalry--calls out Sanji for simply not running away from Kalifa, instead just accepting that he's going to get the shit beat out of him, and possibly die.
So there's a pattern of self-destructive behavior. Sanji repeatedly puts his life on the line when he doesn't need to in order to preserve the lives and dreams of the people he loves. Even him constantly simping over Nami and Robin falls a little into this category, because if either of them told him to take a long walk off a short pier I have no doubt he'd comply. It's that same extreme willingness to sacrifice anything and everything for the people he cares for that we see in Baby 5, except Sanji was fortunate enough to not be surrounded by people that encourage these worst impulses of self-destructive behavior. As he says here in Thriller Bark, he's just the cook. Luffy can always just find someone else.
(The glory of Whole Cake Island being Sanji realizing, no, Luffy can't, and he won't).
And it is finally on WCI that get to the heart of why Sanji is like this with yet another episode of putting his own dreams and happiness aside for the sake of others, and not until Wano that we finally see him take the first steps toward asking others for help instead of passionately throwing his life away when he doesn’t need to.
When Zoro first offered his head to Kuma, the prominence of his dream was first and foremost. Notice that Sanji never mentions the All Blue. One Piece is a series that places the pursuit of one's own ambition above all else, even if that ambition is selfish. Sanji hasn't yet learned to be selfish, so Zoro knocks him out and ends up being the one to accept Luffy's pain. Sacrifice isn't sacrifice if the person doesn't value what they're giving up, and right now Sanji clearly doesn't value his own life compared to the rest of the crew.
Next chapter Oda will speak through Brook to confirm that Sanji's willingness to give himself up wasn't foolish or stupid. It's just that he's missing a piece of the puzzle, and that's not something he'll have for a long time yet.
