In a fanfiction, it could be that the writer wants to show Cho's character flaws. Maybe she's a bit more self - centered than she seems in the books and cheats on Harry because she meets someone who she thinks can offer her more, like better social status among the students or more exciting adventures. Also, it could be a way to explore Harry's reaction to betrayal and how it would shape his character further.
One reason could be that Cho is still hung up on Cedric. Even though she starts dating Harry, her feelings for Cedric might resurface and lead her to cheat. Another possibility is that she feels pressured in her relationship with Harry due to all the attention they get at Hogwarts and looks for an escape with someone else.
Well, perhaps in the fanfiction, Cho cheats on Harry because she gets influenced by some dark magic or a charm that makes her act out of character. Or maybe she starts to feel that Harry is too preoccupied with fighting Voldemort and not giving her enough attention. Additionally, it could be that she misinterprets her feelings for Harry as just a passing infatuation and when she meets someone new, she acts on her new - found 'feelings' without thinking clearly about the consequences to her relationship with Harry.
Chapter 783, "I'll Fight it Out with You Rich People!", is the clearest early answer because the buildup finally cashes out in a real clash. A useful checkpoint is the way the scene starts with As soon as Qingyang finished speaking, everyone's mentality in the...
Chapter 692, "My Ideal Type is Assistant He", is the clearest early answer because it turns Do's situation into a concrete plot event. The chapter turns that into something concrete by moving from Zhao Lang's gaze stayed fixed ahead as he gripped the steering wheel...
Chapter 653, "Everyone's Champion", is the clearest early answer because it turns Wilderness King's situation into a concrete plot event. Early on, As dawn broke, "Wilderness Survival 30 Days" released a notice on....
Chapter 845, "Turns Out He is the Body Double!", is the clearest early answer because it turns Uncle Zhao's situation into a concrete plot event. A useful checkpoint is the way the scene starts with During these two hours of waiting for the results, the restaurant was...
Chapter 808, "Dare to Spank Him?!", is the clearest early answer because it turns Dare's situation into a concrete plot event.
A practical answer is Chapter 16, because it turns the idea into a concrete scene. The chapter opens with Just as the words fell, Jiang Shixu swiftly cast his eyes downward., adds pressure through Smelling the pleasant jasmine scent coming from her, his tense nerves..., and ends on Jiang Shixu.
Chapter 687, "How Did She Die Again?", is the clearest early answer because it turns Yin Le's situation into a concrete plot event.
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Chapter 746, "They Are Each Other's", is the clearest early answer because it turns Other's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The fervor in that young man's eyes was something she would never...
Chapter 820, "He’s actually even harder to coax than Buchen!", is the clearest early answer because it turns Waa's situation into a concrete plot event. Early on, Today, he was particularly cautious with his eating, even sipping his....