It's the job of the captain to pick the crew, and the duty of the first mate to test them for worthiness. How Zoro came to accept each of the straw hats as nakama.

Post Drum Island-Arc. Chopper likes everyone in his new herd. Although he's not quite so sure about Zoro, who smells like metal and blood.

From the other end of the ship, panting as though he has run from the depths of his workshop, Usopp is the next to let out a scream: “Why is there a SHARK ON THE DECK!?”

Imagine person A making person B a friendship bracelet, expecting person B to never wear it, but when it’s given to them person B puts it on and is rarely seen with it off. A group of marines charge, Zoro slices through them, and in that instant Sanji feels his own eyes grow wide. Because there, on the arm now outstretched towards him, steel glinting in hand, is the stupid bracelet he’d given Zoro. The bastard is actually wearing it.

"Zoro!" Luffy gave him a happy grin, looked out at sea...swung his head about to give his first mate a slightly longer look centered on the latter's new bust line, then back up at Zoro's face as he waved excitedly. "Hurry up! Nami says the log pose will set here if we wait too long, and then we won't be able to sail to Merman Island because we'll be set on another course. Hey, do you know your clothes are too big?"

You know how this is going to go.

Zoro and Sanji's second time. The less said about the first...