A sense of powerlessness can also be a theme. When he's faced with situations where he can't immediately solve a problem or protect those he cares about, Harry might get pissed off. This often leads to him finding new ways to gain power or take control in the fanfiction.
Another reason might be related to his family. In some fanfictions, his parents' story or his relationship with his family members could be distorted. Harry has a strong connection to his family, especially his parents, and any wrong portrayal of that could make him pissed off.
Another good one is 'When Harry's Temper Flared'. Here, it starts with a small incident in the Gryffindor common room that snowballs into a major event where Harry gets extremely pissed off. The writer focuses on Harry's relationships with his friends during this time of anger and how they react to his outbursts.
Sasuke could be the cause. If Sasuke were to betray Naruto again in the fanfiction, it would surely make Naruto pissed off. Their relationship is complex and Sasuke's actions have a big impact on Naruto.
It can make the story more intense. A pissed - off Harry Potter is likely to take more drastic actions. For instance, he might be more aggressive in his fight against the dark forces, which can drive the plot forward quickly.
Sasuke could make Naruto pissed off if he goes back to his old ways of seeking power and revenge. Naruto has worked hard to bring Sasuke back to the right path, and if Sasuke strays again, it would surely anger Naruto.
He can turn into a more determined version of himself. Once pissed off, Harry may set his mind on finding the truth or getting revenge. His stubborn streak gets stronger. He won't be easily dissuaded from his goals. This can lead him to take risks that he wouldn't normally take. In some fanfiction, this new - found determination can be both a good and a bad thing. Good in that it helps him solve problems, but bad in that it can put him in more dangerous situations.
One common theme is revenge. When Naruto is pissed off, often he's seeking revenge against someone who has wronged him or his friends. For example, if Orochimaru did something to harm Sakura, Naruto would be pissed off and his revenge - driven actions would be the focus. Another theme could be self - discovery. His anger might lead him to discover new powers or a new side of himself. Like his anger might unlock a hidden bloodline limit that he didn't know about before.
A new, powerful and arrogant villain introduced in the fanfic could be the cause. This villain could constantly mock Naruto and his ideals, and try to undermine his efforts to protect the village. Such blatant disrespect would make Naruto really pissed off as he is very protective of his home and the values he stands for.
To find out the author of the 'harry gets pissed fanfiction', you could try searching on fanfiction platforms. Look for the story and check the details page. Usually, the author's name or username will be clearly stated there.