One possible reason could be misunderstandings. Maybe Hinata says or does something that Naruto misinterprets in the fanfiction. For example, she might seem to reject his feelings in a really harsh way that breaks his heart.
He might isolate himself. Naruto has always been a social person, but the heartbreak could be so intense that he just wants to be alone. He might go to his favorite training spot and just train by himself for days, trying to forget the pain through physical exertion.
There's a fanfic titled 'Hinata's Comfort for Naruto'. In it, Naruto is heartbroken after a major loss in the village. Hinata, who has long admired him from afar, steps in. She first helps him by taking care of his daily needs like cleaning his messy apartment and doing his laundry. This simple act of kindness makes Naruto realize that someone truly cares. Then Hinata shares her own experiences of feeling alone and how she overcame it, which gives Naruto the strength to start anew.
One key moment could be when Hinata simply sits beside Naruto when he's at his lowest. Just her presence can be a huge support. Another is when she gives him a small gift, like a handmade charm with a message of hope. It shows that she's thinking about him and wants him to feel better.
In this kind of fanfiction, Naruto being heartbroken by Rias could be due to a variety of reasons. Maybe Rias betrays Naruto in some way, like choosing someone else over him in a crucial situation. Or perhaps there are misunderstandings between them that lead to Naruto's heartbreak. It could also be that Rias has to follow some rules in her world that go against Naruto's expectations, causing their relationship to shatter.
One aspect could be that the fanfiction shows Naruto's heartbreak in a very visual and emotional way. For example, he might see his parents in a dream - like state and then they disappear, leaving him feeling more alone than ever. It's a way to dig deeper into his character's vulnerability, which was somewhat masked in the original series by his determination.
Betrayal is another big theme. When someone Naruto trusts turns against him, it breaks his heart. It often leads to him feeling isolated and having to re - evaluate his relationships. For example, if Sasuke were to betray him in a more extreme way in these fanfics. And it's not just about the act of betrayal but also the emotional aftermath that Naruto has to deal with. He has to face his own feelings of hurt, anger, and confusion. Sometimes, he might even start to doubt his own judgment in trusting people.
One great Naruto heartbroken crossover fanfiction is 'Naruto in the World of Bleach: A Broken Heart's Journey'. In this fic, Naruto is transported to the Bleach universe and his experiences there, along with missing his friends from his own world, lead to a heartbroken state. It has great character development and an interesting plot that really delves into Naruto's emotions.
One possible reason could be the loss of a dear friend. Naruto values his friends deeply, and if he were to lose one, it would break his heart. For example, if Sasuke were to die or turn against him completely in a way that Naruto couldn't fix, it would be a huge blow.
One possible reason could be a failed relationship. In the DxD fanfiction world, Naruto might fall in love with a character, but due to differences in their goals or the interference of others, the relationship fails. For example, if he falls for a devil girl who has to follow the orders of her clan which oppose her being with Naruto. Another reason could be the loss of a dear friend. Maybe one of his newfound friends in the DxD universe betrays him or gets killed, leaving him heartbroken.