Well, it could be that Naruto discovers an ancient artifact that contains Hashirama's chakra. When he touches it or unlocks its power in some way, the chakra gets transferred to him. For example, the artifact might be hidden in a secret location and Naruto stumbles upon it during one of his adventures.
Perhaps the origin of the chakra chains lies in Naruto's connection to the tailed beasts. Since he has a unique bond with the nine - tailed fox, the chakra chains might be a manifestation of that bond, combined with his own latent Uzumaki powers. It's possible that as he grew stronger and his understanding of chakra deepened, this ability was awakened.
Maybe it has something to do with his Uzumaki heritage. The Uzumaki clan was known for their strong chakra, so it could be a latent ability passed down through the generations.
It could be the result of some kind of special training. Maybe he met a hermit who taught him a unique technique to perceive chakra in a visible way. For example, this hermit might have had him meditate in a special chakra - rich area and gradually opened his eyes to the world of visible chakra.
Naruto's chakra mastery affects his relationships in multiple ways. In his friendship with Shikamaru, his chakra control allows him to keep up with Shikamaru's strategic plans during battles. This makes their teamwork seamless. With Hinata, Naruto's powerful chakra makes him more of a hero in her eyes. His chakra also gives him the ability to sense when Hinata is in danger and come to her rescue. Among the villagers, his chakra mastery earns him more respect, which changes the way they interact with him from being skeptical to being admiring.
Naruto's dense chakra could make his friends either jealous or in awe. If he can do things with his chakra that others can't, some might envy his power. But his true friends like Sasuke and Sakura would be in awe and see it as a way to strengthen their team. For example, Sasuke might see Naruto's dense chakra as a new challenge to keep up with, which could improve their friendly rivalry.
He might gain the ability to control wood - style jutsu more effectively. Since Hashirama was known for his powerful wood - style, Naruto could use it to create huge forests for defense or to trap enemies. For instance, he could grow trees instantly to block incoming attacks.
In fanfiction, Naruto's yellow chakra may be more potent. His normal chakra is his basic energy, but the yellow chakra could be a special form. It might be stronger, allowing him to perform more powerful jutsus or move at greater speeds.
It might be a result of some special training or encounter. For instance, if Naruto had a unique experience during a mission or while training with a new mentor, this could have triggered the unlocking of the chakra chains. It could be that he tapped into a new source of chakra or learned a new technique that enabled this ability.
Naruto unlocking chakra chains could be a turning point in the fanfiction's plot. It could mean that he has a connection to some hidden or forgotten part of his heritage. This new power could attract the attention of the villains in the story. They might see him as a greater threat or as a valuable asset to their own plans. Consequently, the whole story could shift from Naruto being on the defensive to him being actively hunted or courted, which would be a major plot shift.