WARNING: The following contains major spoilers for Boruto: Naruto Next Generations #35, on sale now.
In the Naruto franchise, one of the most appealing arcs was the evolution of the relationship between Naruto and the Nine Tails "Demon Fox" stored inside him since birth, Kurama. It was the reason he was shunned by Konoha and why he found life tough as an orphan outcast in his formative years.
Eventually, they started to understand each other more as Kurama allowed the teen to dip into his own powers. By the time the franchise ended, they more or less became one, which allowed Naruto to fulfill his dream of becoming Hokage. Still, it seemed like it was just a mutually beneficial relationship where Kurama needed an avatar and Naruto needed to level up to save the ninja world from Kaguya and her minions. However, courtesy of the Boruto manga, we finally know why both entities bonded to become the ultimate savior these shinobi villages needed.
Kurama was stored inside Naruto following a battle against Obito, who along with Madara, entranced and unleashed it on Konoha. Minato and Kushina, Naruto's parents, subdued the fox and stored it inside their newborn baby before they died, as Naruto's bloodline was the only thing that could contain such a powerful beast. As Naruto grew up, Kurama came off like a corrupting force that tried to turn him into a weapon, but he'd soon realize Naruto wasn't like other humans. He was a being of care and compassion, which forged their friendship and turned Kurama away from his bloodthirsty ways.
And as Kurama now explains to Kawaki years later, he saw it as more than a friendship. To him, Naruto is a son. In fact, Naruto was also the closest analogue to the Sage of the Six Paths -- the father of all shinobi and the man who looked after all nine Tailed Beasts centuries ago when they were allowed to roam free.
In this chapter, we see Kawaki, thanks to a chakra infusion from Naruto, recuperating at the Hokage's home and training so he doesn't follow the evil road Kara and its boss, Jigen, mapped out for him as a weapon. His stay there allows Kurama to surprisingly speak to Kawaki and he explains that while Naruto was a vessel (just like how Kawaki is to Jigen), he was more than a means of surviving. Kurama felt that battle after battle, Naruto actually came off like a caretaker towards him. What shocked the fox most was the teen didn't want to lose Kurama to the Akatsuki, not because he felt he'd lose power, but because he didn't want Kurama to be taken advantage of. This is where the fox realized he had an owner he could trust and grow to love.
Kurama mentions that Naruto's pained childhood, similar to the genetically engineered Kawaki, drew him to the boy, and his respect for Naruto grew as he excelled in the face of adversity. As their journey continued, Naruto began to see Kurama more and more as a best friend, and Kurama quickly felt that he was a father-figure, even dispensing advice when they met on the astral plane in the Shippuden era. Coincidentally, Naruto's persistence and resilience is something he also sees in Kawaki, and urges him to pursue his dreams relentlessly too. As part of the pep talk, he reminds Kawaki (whose name means "dried" in Japanese) that his heart isn't parched after all.
Kurama recalls how he grew and matured from Naruto's kindness. When he realized he could become a better beast with Naruto, he knew he found the perfect vessel, which is why he's never wanted out since. So while the Naruto series addressed this on the surface, the Boruto manga goes deeper than ever and finally gives us the truth which Kurama kept close to his heart.