His debut arc kind of goes out of its way to point out the mindset difference between him and Zoro/Luffy.
First of all, before I point them out, do note that I'm not saying that these are most important thing or primary requirement for CoC. Hell, we don't even know what they are. But I'm pointing out how those two were set apart from Sanji in the factor of their conviction, will and pride.
Additionally, I do think Sanji understood where they come from eventually. But stuff like this is part of who you are- it comes from get-go, and isn't something that's taught. Both Luffy and Zoro were like this even as kids, and it's an inherit trait that's carried on- much like what's described as CoC.
When Zoro points out his ambition, Sanji is the first to hop on and try to diminish it saying it's only asking for death:
Later on when Zoro literally chooses to die instead of backing off against Mihawk, Sanji straight up doesn't even understand why:
And even screams at Zoro to live, even if it means abandoning and giving up on his ambitions:
Later on, we see literally same qualities being mentioned in Luffy from Zeff, while Sanji listens intently:
Note that this
doesn't just apply to ambitions, but convictions in general- yet Sanji doesn't understand that concept, still, and is visibly speechless at Judge's explanation:
Later on, Sanji even laments on what Zeff said, and thinks of both Zoro and Luffy:
Even if he understood later on, it's clear that Sanji's concept of sheer, never-back-down conviction was not even close to Zoro or Luffy's. He didn't even understand it at first, or why they did what they did until Zeff explained it- and though he did realize later on, it won't randomly give him those traits.
Also, there's some sort of implication in WCI, too:
Luffy seems to agree with most of this stuff and considers it good, which it is.
Having those "useless" emotions that cause him to protect others isn't a bad thing. And there's nothing wrong even if he's a Germa failure or can't use his skin as shield. Or if he's unfit of a soldier. To Luffy, and in general, those things don't make Sanji a better person, so it's all good.
But Luffy doesn't particularly disagree with the notion of Sanji having a weak mind or lack of pride.
Again, you're free to disagree and think Luffy interpreted as weak mind in regards to being emotionless or something. And the pride referred to here specifically means some bad-people stuck-up pride. And that's okay. This was a very small and least relevant part of my post anyway.