You still don't get it, do you? Even weak people can sacrifice their life for their loved ones. How many can sacrifice life for their dreams? Thats what separates conquerors like Zoro-Luffy to an average Joe like Sanji.
Except for some their conviction has to do with protecting the dreams of their loved ones at various points.
That's Ace with Whitebeard for example, where he demands to be left for dead and that the WB pirates don't start a war for his sake. That's also Ace wanting to make Whitebeard the Pirate King, and fighting for Whitebeard's dream.
It's the same as Sanji doing it for Zeff and not being willing to do it for himself because of the debt he felt towards Zeff.
Basically Sanji did not value doing something for himself, so his will power and conviction was all directed towards Zeff. That's why he could look down on others he'd think as selfish for acting for themselves, and foolish to get killed for that.
It's all explained in the same arc, where Zeff describes CoC
and let's us know that Sanji also has it in him and must no longer stiffle it and act for himself