Of all the interpersonal relationships to form on the Thousand Sunny in One Piece, the most wholesome of all has to be Zoro and Chopper's. Someone unfamiliar with the series might not expect the badass, battle-hungry swordsman and the cute, high-strung reindeer doctor to have an unspoken father-son relationship, but the truth speaks for itself.
When looking at their placement in the Straw Hat Pirates, it makes complete sense this type of relationship would form. Chopper was only 15 when he joined the crew in One Piece, quite inexperienced in the outside world. The only people who accepted him were Dr. Hiriluk, Kureha, and this ragtag pirate crew he'd just met. So seeing this fearless swordsman named Zoro continuously take on droves of opponents at once, it makes complete sense Chopper would look up to him. Let's look at the progression of this lovable duo over the course of One Piece.
Multiple moments throughout One Piece have demonstrated how much Chopper looks up to Zoro -- and how Zoro looks out for Chopper. Throughout the pre-time skip, Chopper will often cling to Zoro for security when he's scared or worried, and Zoro will give his buddy some well-meaning tough love.
A classic example is in One Piece's underrated Long Ring Long Land arc, where Chopper is chosen to join the Foxy Pirates after the crew loses its first game. Chopper is understatedly upset and starts to cry -- he only became a pirate to join Luffy's crew. Zoro interrupts, saying Chopper has no one to blame but himself. They lost and nothing can change that, so if Chopper really is a man, he should act like one and believe in his Nakama by seeing this fight through to the end. Immediately, Chopper wipes the tears away and stands up straight so as not to disappoint Zoro's belief in him.
Zoro is not the type to sugarcoat or beat around the bush in One Piece. His straightforward approach can seem cruel, but it's really just his way of showing he cares. This resonates with Chopper enough that he sees Zoro as a model example of the pirate he wants to be.
This could stem from a conversation between the two while traversing the desert in Alabasta. Chopper's only been with the crew for a short time at this point, and it isn't quite what he expected. He asks Zoro why he joined Luffy and his comrade responds, "It just sort of happened that way." Zoro points out the Straw Hats don't seem to have the same level of teamwork as other crews, but that's not a bad thing. Everyone in the crew joined for their own goals. When they fight they all give it their best shot, and when they fall is when they rely on each other. Through that approach, a different reason has emerged for why each of them is part of this crew: to see Luffy become King of the Pirates.
It's an excellent scene that shines a light on how Zoro sees the world around him, why Chopper sees him as so reliable, and by extension, the relationship the crew has with its captain. This scene is actually filler content from the One Piece anime, but it could easily count as canon material.
The nature of that scene comes back into play later on in Fishman Island. When the Straw Hats are fighting the New Fishman Pirates, Zoro comments on Chopper's new Heavy Point form, saying he looks more "monstrous these days." Chopper replies that he doesn't mind -- he's fine with being a monster for "Luffy's sake" to which Zoro smiles in agreement.
Even today, fans still get Chopper-Zoro moments in One Piece, with the most recent example being the reindeer treating the bandage burrito swordsman in Onigashima.
One Piece fans have always loved Chopper and Zoro because it's so sweet seeing the stoic swordsman being protective and looking out for the chibi reindeer. Conversely, it's just as fun seeing this adorable reindeer think the scary battle-hungry swordsman is the coolest guy on the ship. Their dynamic is fun and different from all the other relationships in the crew, and that has led to mountains of fan art of the two being their adorable selves.