Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
It could be a result of a powerful enemy's attack. In Wave, there could be a hidden foe with a really strong jutsu that Naruto couldn't fully dodge, and his arm got caught in the crossfire.
Perhaps Naruto underestimated an opponent in Wave. He's sometimes overconfident, and if he rushed into a fight without proper preparation, the enemy could have taken advantage and injured him severely, resulting in the loss of his arm.
Another possibility is that Naruto and Yang have a special bond. This bond could be formed through shared experiences, like they both went through a difficult training together or survived a life - threatening situation. Because of this special connection, Naruto feels indebted to Yang or simply values Yang's life more than his own arm.
Maybe he starts to train with one - armed techniques. He could learn from other one - armed ninjas if there are any in the fanfiction world. His friends would also play a big role. They might encourage him to keep going or help him in his training.
In the context of the fanfic, it could also impact his relationships. His friends might be overly protective, which Naruto may not like. Or, he could inspire others with his determination to keep going despite his injury. It might also change how he is perceived in the ninja world. Some may see him as weakened, while others could view him as even more of a hero for his ability to persevere.
This event would be a huge setback for Naruto. He's always been a very physical fighter, using his clones and his rasengan. Losing an arm would force him to adapt. Maybe he'd focus more on strategy and less on brute force. It could also make him more empathetic towards others with disabilities in the ninja world.
One possible reason could be an enemy attack. A powerful and malicious foe might target Naruto's family to get back at him, and his wife could unfortunately become a casualty in the battle.
Well, in some 'naruto loses arm fanfiction', it might be a result of a forbidden jutsu gone wrong. Naruto might be exploring a new and dangerous form of power, and the price is his arm. This would then set off a chain of events where he has to find a way to either get his arm back or adapt. He could also face discrimination from some in the ninja community who see him as weakened, but he'd prove them wrong by his unwavering spirit.
Well, he might face difficulties in combat. Without his arm, his fighting style would have to change drastically. He may rely more on his ninjutsu and less on physical attacks. Also, his daily life activities like eating or holding things would be a struggle.
One possible plotline could be Naruto having to relearn all his ninja skills with only one arm. He might struggle at first but then discover new ways to perform jutsus, like using his legs more for balance and chakra control. Another could be his friends trying to find a way to restore his arm, leading them on a dangerous journey to find a rare artifact or a powerful healer.
With his friends, it might strengthen their bonds. They would be more protective of him and help him in his daily tasks. For example, Sasuke might become more empathetic towards Naruto's situation. His relationship with Hinata could also change. Hinata would be very concerned and would constantly be by his side, offering emotional support.
Well, losing an arm would definitely have a huge impact on his ninja abilities. His ability to perform hand seals would be severely hampered. For example, complex jutsus that require multiple and quick hand seals might become impossible for him to execute. Also, his balance in combat could be affected as he's used to having two arms for movement and defense. He'd probably have to rely more on his legs and other non - arm - related skills.