I was just thinking about the Luffy and Zoro dynamic, and why I think their bond is different from the other SHs.
As a captain, we have seen Luffy suffer for his friends. He also wants to take away their pains and burdens. For example, he pushes himself to near death in his fight against Lucci just to show Robin that she had friends fighting for her.
Zoro is the only SH that has suffered with Luffy. Luffy allows this because he knows Zoro is tough enough to handle any suffering. Hence why he asks Zoro not to fight back when Bellamy harasses them. Has he ever asked another crew member to suffer with him?
It’s also why Zoro was the only other SH who fought Kaido and Big Mom with Luffy. Luffy trusts that Zoro is able to handle anything that is thrown at him. Both of them went through the ringer during Onigashima, and I like how they were treated together. They suffered together and they recovered together.