One possible reason could be unrequited love. Maybe Natsu has feelings for someone who doesn't feel the same way about him. This could lead to his heartbreak as he realizes that his love is not reciprocated.
He might isolate himself at first. Natsu could withdraw from his friends and normal activities, needing time alone to process his pain. This is a common way for people, or in this case, characters, to deal with heartbreak.
A theme that often pops up is the idea of growth. Heartbreak can be a catalyst for Natsu to grow as a character. He might become more empathetic or learn to be more cautious in relationships. Also, the theme of forgiveness can be present. He may have to forgive the person who caused his heartbreak to move on.
One popular 'natsu gets heartbroken fanfiction' could be 'A Broken Heart's Journey'. It delves into Natsu's emotional turmoil after a betrayal from a close friend. The story shows his struggle to come to terms with the pain and how he gradually tries to heal.
Well, perhaps in the fanfiction, Ash is betrayed by a group of friends he trusted deeply. They might have used him to get access to some rare Pokémon or some important information, and then they turn their backs on him. This would be a huge heartbreak for Ash, who is always so loyal to his friends. Or it could be that he fails to achieve a major goal like winning a big Pokémon tournament due to some unfair circumstances, and he feels heartbroken as all his efforts seem to be in vain.
A plot twist could be that the person who breaks Harry's heart has a secret identity. They could be a spy from an enemy group, but during their time with Harry, they actually fell in love too. However, their loyalty to their original group makes them break Harry's heart. Or perhaps the heartbreak is due to a time - traveling incident. Harry meets someone from the future who has to leave him in order to preserve the timeline. And also, it could be that the one who breaks Harry's heart is actually under a spell that makes them act out of character, and when the spell is broken, they try to come back to Harry but it's too late.
It could be that there is some misunderstanding between them. Say, Misty misinterprets something Ash does as him not caring about her anymore. Ash, being clueless about her misinterpretation, doesn't correct it in time. Then Misty distances herself from him, and Ash is left heartbroken when he finally realizes what has happened.
A great natsu heartbroken fanfiction is 'When Natsu's Heart Broke'. It explores a situation where Natsu is deceived by someone he trusted, leading to his heartbreak. The story is full of his internal struggles and how he tries to move forward.
It could slow him down. Ash might be so distraught that he doesn't focus well on training his Pokemon. His usual enthusiasm for battles and catching new Pokemon might wane, as his mind is preoccupied with his broken heart.
Well, it could be due to the failure to protect the village. Naruto has always been about protecting Konoha. If something really bad happened, like a major attack that he couldn't prevent and many lives were lost, he might feel so much guilt and heartbreak that he decides to leave. He would feel like he failed his purpose.