In some cases, the girl might feel neglected by the boy. She could be seeking more attention from someone new. Or perhaps she has been influenced by others around her who don't think the boy is good enough for her. It's also possible that she has some personal issues that she hasn't dealt with properly and ends up making the wrong choice which betrays the boy.
One reason could be greed. Maybe she sees a chance for a more luxurious life with someone else. Another could be that she has fallen out of love but is too cowardly to be honest about it.
She may be attracted to someone who she thinks has more exciting qualities. For example, if the boy is a homebody and she meets an adventurous type. Also, she could be under pressure from her family or society. If her family doesn't approve of the boy, she might choose to betray him to please her family. Or if society has certain standards that she thinks the boy doesn't meet, she could be swayed to betray him.
Chapter 119, "Wen Jiaojiao is Poisoned!", is the clearest early answer because it turns Xiao Wu's situation into a concrete plot event. It opens with What is Princess Wen talking about? and closes on For some reason, he always felt there was a mysteriously calming....
Chapter 3, "Madam, the eldest young master has been missing for over half a month", is the clearest early answer because it turns Capital's situation into a concrete plot event. It opens with Qiao was still annoyed at the attitude of this unknown girl when she...
Chapter 348, "Zhu Ling in Dire Straits, Eight Camps Martial Contest, Betting Five Years—East Ridge Has Fallen!", is the clearest early answer because it turns East Ridge's situation into a concrete plot event. The chapter turns that into something concrete by moving from Moao Calendar, Year 131, September 15 Beining Mountain, Zhu Ling... to Therefore, for Zhu Ling Camp, with a population of around 60 to 70....
Chapter 288, "The Old Master Already Knows", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Jiang Li and Zhao Lang were lifted onto stretchers and taken to...
Chapter 341, "Moao's Nine Towns, the Inspection System and Zhenyu, Li Hu and Hong Yuru", is the clearest early answer because it turns Xing River's situation into a concrete plot event. It opens with The entire Moao Mountain Range stretches from northwest to southeast. and closes on The powerful northern Three Towns block the north..
Chapter 334, "Hanqiong's Seven Divisions, Moao Mountain, Beishuo Revisited 5", is the clearest early answer because it turns Twin Dragon Mountain's situation into a concrete plot event. A useful checkpoint is the way the scene starts with According to Fan Kui, let's first explore northward together to see... and follows through with Beining Mountain, seven camps, judging by this kid's cultivation and....
Chapter 79, "Finally, I'm not the last one anymore!", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event. Early on, Jiang Li leisurely said, "I bought this weeding machine, and....
Chapter 422, "The Watch Was a Gift from Him", is the clearest early answer because it turns Team's situation into a concrete plot event. Early on, Seeing that no voice came from the watch to stop them, the captain of....
Chapter 109, "Not One Deserves to Stay on His Friends List!", is the clearest early answer because it turns Is Jiang Yan's situation into a concrete plot event. It opens with And another thing," Jiang Li said, taking out a tissue to wipe her...
Chapter 345, "Luo Ming, Mirror Valley Defense Commander, Beast Tide, Great Xia's 6 Armies", is the clearest early answer because it turns Ice Trees's situation into a concrete plot event. It opens with Great Xia, Year Seven, September Ninth, the latter half of the night... and closes on The Ice Tree takes advantage of its role as a staple, otherwise, the....