Well, it could really mess things up. Naruto's relationships are often based on his big heart and passion. If he seals his emotions, his interactions would become more mechanical. People who love him for his energy, like Iruka, would find it hard to connect with this new, emotionless Naruto. He'd be like a different person, and his relationships would have to start from scratch in a way.
There is a possibility that it was an ex - shinobi who had retired from active duty. This person had amassed a collection of seals during their career and now made a living by selling them to other ninjas like Naruto.
Well, considering Naruto's own experiences and the variety of seals in the Naruto universe, he could sell communication seals. These would allow ninjas to send messages over long distances. He might also sell seals that are related to elemental jutsu, perhaps ones that enhance or modify fire, water, wind, earth or lightning techniques.
Perhaps Naruto was in a situation where he had made a deal. For example, he promised to pay for the seals in exchange for some information about a missing nin or a secret technique. He values knowledge and power, and if paying for the seals was part of getting closer to achieving his goals, he would do it.
One possible reason could be to protect her. Maybe she has some special power that attracts dangerous enemies, and Naruto, being the strong and protective character he is, seals her power to keep her safe from those who might misuse it.
Well, he would probably become much more cold and detached. He might lose his drive to protect his friends and the village in the same passionate way. His actions would be more mechanical, lacking the warmth and empathy that used to be his trademarks.
Well, Naruto in fanfiction often shows his emotions verbally. He's not one to hold his tongue. So, he might passionately declare his love for Hinata in a really sappy and over - the - top way that makes everyone around him roll their eyes but also feel the warmth of his emotions. Also, when he's sad, he could be shown as isolating himself from the group, which is a clear sign of his inner turmoil.
The seals could also be a plot device for a darker storyline. Maybe they were placed by an unknown enemy to control Naruto or use him as a weapon. In some fanfictions, the seals start to affect Naruto's mental state, causing him to have inner struggles between his normal self and the influence of the seals.
It could be for strategic reasons. In the world of ninjas, showing too much emotion can be a weakness. Naruto might think that by sealing his emotions, he can become a more effective ninja and better protect his village.
Perhaps in the fanfiction, Naruto accepts his dark emotions with the help of his friends. His friends, like Sasuke, Sakura, and Kakashi, might encourage him to face his inner demons. They tell him that it's okay to have dark emotions and that everyone has them. Naruto then starts to accept his own, knowing that he doesn't have to face them alone. His friends' support gives him the confidence to embrace his whole self, including the darker aspects of his emotions.