One Piece is one of the longest-running anime and manga series. The manga started publishing in 1997 and the anime came out in 1999. The story is given by Oda Eiichiro, who quickly stole the hearts of the public with his amazing art and story that tickles our fantasy.
The story revolves around Monkey D. Luffy, who gathers a bunch of crew members to set sail and explore the world.
While doing so, he also wishes to find the legendary treasure named "One Piece" and become "King of the Pirates". Luffy sets out on his journey, gathering a group of people to join his crew.
The first person Luffy comes across is Zoro. Zoro is Luffy's first mate and right-hand man. Their friendship excels beyond the boundaries of a captain and a crew member. Luffy is the reckless one while Zoro keeps him in check. Here, we will discuss why Zoro is so loyal to Luffy.
Why did Zoro Join the Crew?

Before joining the crew, Zoro was a pirate hunter, infamous for his three-sword style technique that no one in the world seemed to possess. Zoro was captured by the marines for punching Helmeppo whose father, Morgan was a marine captain at that time.
Morgan was used to abusing his powers and bullying the weaker ones. Helmeppo was no different from his father. The only reason Zoro punched him was to stand up for a little girl.
After being captured, Morgan made a deal with Zoro that if he were to survive without food for a month, he would spare Zoro, the girl, and her mother. However, he had no intention of keeping that promise and wanted to execute Zoro instead.

After finding out that he had no chance but to be executed, Zoro accepted Luffy's offer to join his crew. Zoro had to accept that offer for achieving his dream of becoming the greatest swordsman in the world.
What Promise did Zoro make to Luffy?

In the Bariate Arc, the Straw Hat pirates come across Dracule Mihawk. He is one of the seven warlords and is said to be the strongest swordsman in the world. Zoro challenges Mihawk and loses easily. The difference in their powers was simply too much.
However, Mihawk sees potential in him and spares his life. He also encourages him to hone his skills and face him again when he is stronger. After his fight, Zoro lifts his sword in the air and apologizes to Luffy for making him worry. He also says that it would be a real dilemma for Luffy as Pirate King if Zoro was not the world's greatest swordsman.
Fighting against his pain, Zoro tearfully vows to Luffy that he will never lose any fight until the day he defeated Mihawk and becomes the world's greatest swordsman. He acknowledges Luffy as the future King of the Pirates and asks if the latter has any problem with that, to which Luffy cheerfully denies.
Why is Zoro so Loyal to Luffy?
Although every Straw Hat member respects Luffy as their captain, Zoro has shown the most faith in his captain time and time again. At first, joining Luffy's crew was a stepping stone for Zoro in achieving his dreams. However, as he gets to know Luffy better, he sees him stopping others from hurting innocent people.
Luffy also encourages Zoro to fulfill his dream. The biggest turnover was Zoro's fight with Mihwak after which, he truly pledges loyalty to Luffy. Even though Zoro still dreams of being the strongest swordsman in the world, he now wants to do that for the sake of the future Pirate King.
Not only that, but Zoro puts Luffy's dream above his. It was proven in the Thriller Bark Arc where he says that he would be fine even if he doesn't achieve his dream and dies first. However, he must do everything he can to make Luffy the King of the Pirates.