I'm really curious about Sanji's 'past past' too! Other Strawhats must also have their own significant history yet to be unfold, but Sanji's the only one who gives me the feeling that he's intentionally hiding his past(Maybe you'll bring this up in the Skypiea arc much much later). And young Sanji being dead serious about the killing business.. could it possibly be related to him originating from North Blue? From what I have gathered until this point that particular sea doesn't seem like a nice place to live, does it? (Cold weather, forbidden to dream, Doflamingo gathering many of his surbordinates from there, Law's past, etc.) Anyway, like you've said, so many questionsssss.

I wouldn't say that Sanji's (known) past is the most traumatic out of all the Strawhats. Maybe it's even safe to say that his past is the least tragic since he didn't lose anyone precious to him(well he did lose the Orbit cooks but you can't tell if he was really attached to them). But I think we can all agree that along with Brook, Sanji is the person who knows best how terrible and overwhelming the vast ocean can be. Hell, you can even say that the ocean is Sanji's life itself. Every Strawhat's past life was interlinked to the ocean in one way or another, but Sanji's the only one who actually grew up on a freakin ship. Even his dream is discovering a mystic ocean. I personally find it interesting that Sanji and Brook were the only ones to become Luffy's nakama on a boat, not on land. They're also the ones to provide the main source of entertainment - food and music - whenever the Strawhats throw up a party. Those who suffered the most by the cruelty of the ocean providing the source of joy to their crewmates..did Oda intend this or is it mere coincidence?

And it's also worth noting that Luffy, Zoro and Sanji(a.k.a. 'the Monster Trio') are the only ones who voiced their desire to become stronger for others when they were young. Other Strawhats could've thought the same thing but I don't remember their younger selves saying this out loud like the monster trio did when they were kids so I think that's pretty interesting too. Did Oda intend this as well?

To quote Harry Potter and the Sorcerer’s Stone: “There are some things you can't share without ending up liking each other” being stuck starving on a rock is definitely one of them, and I think this is definitely true for Sanji in addition to Zeff sacrificing for him.

They are who they are but due to their stubborn natures, self sacrificial natures, and anger, Sanji’s largely born out of this incident, it’s difficult for them to come to an understanding that doesn’t involve belittling the other. That being said, I adore who they are and how hard they fight to prove their love without proving their love. Because sometimes that’s just the way things go.

This. You summarized their relationship perfectly! Couldn't have put it any better :D