In the fanfiction, perhaps Sasuke has a hard time expressing his emotions. Sakura might feel unloved or unappreciated in the marriage. She may have expected more emotional connection and when it doesn't happen, regret sets in.
Another aspect could be that Sakura realizes they have grown in different directions. She might have expected a more traditional family life, while Sasuke is still very much caught up in his past traumas and the need for revenge - or in this case, whatever complex motives he has in the fanfiction world. This misalignment of expectations can make her regret the marriage.
Chapter 166, "Daniu Gets Jealous!", is the clearest early answer because it turns Zhou Yi's situation into a concrete plot event. It opens with Only after everyone in the small courtyard had left, did Zhao Lang...
Chapter 423, "A Scumbag!", is the clearest early answer because Nina gets a payoff that changes the stakes. It opens with Are you hurt?" Yu Li asked, panting. and closes on Bone setting is supposed to hurt a little." As he said that, he....
Chapter 351, "Miss Jin Zhen Liu Suqing", is the clearest early answer because it turns Jiang Zhao Yuan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Yan: "....." How does someone manage to praise themselves with...
Chapter 284, "You'll Never Get Rid of Me!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter turns that into something concrete by moving from Bai Lu, also an actress, naturally saw through the insincere...
Chapter 108, "Without accumulating virtue through words, calamities of bloodshed will only increase, not decrease.", is the clearest early answer because it turns Brother Shaojing's situation into a concrete plot event.
Chapter 239, "Although he's cheap, he's also stupid", is the clearest early answer because it turns Shining Transformation Trip's situation into a concrete plot event. The chapter gives readers a real answer by opening on I can't believe it, I thought the live broadcast was over, but here's...
Chapter 425, "Congratulations on Becoming a Substitute Guest!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from Chen Fan looked uneasy at Jiang Yan.
Chapter 154, "He Came, Bringing with Him the Bastard Game", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. Early on, Hahaha, I'm dying of laughter, Lili, do you really have to be so....
Chapter 330, "Why is it her?!", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Of course," Ye Chu pulled a lipstick out of her purse and started...
Chapter 395, "She's Not Trying to Use Under-the-Table Tactics, Is She?", is the clearest early answer because it turns Table Tactics's situation into a concrete plot event.