One common theme is loss. Maybe Lucy loses a dear friend or a precious magical item in the fanfiction, which makes her sad. Another theme could be unrequited love. For example, if she has feelings for someone who doesn't return them, it would cause her sadness. Also, failure in her missions or not being able to protect her friends can be a theme that leads to her being sad in these fanfictions.
Often, themes of betrayal are seen in Fairy Tail Lucy sad fanfictions. If a friend or a guildmate betrays her trust, it would be very sad for Lucy. Additionally, self - doubt can be a theme. She might doubt her abilities as a celestial wizard, thinking she's not good enough compared to others in the guild, which would lead to her sadness. And the theme of separation from her family or her past might also be explored, like memories of her father resurfacing and making her feel lonely and sad.
Chapter 733, "Those People from the Jiang Family Are More Stupid than the Next!", is the clearest early answer because it turns Next's situation into a concrete plot event. The chapter turns that into something concrete by moving from Okay, Sister, you should go to sleep now.
Chapter 670, "Sister Will Support You", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. The chapter gives readers a real answer by opening on Jiang Li, sensing that something might have gone wrong, quickly used...
Chapter 485, "Straight On Clash!", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 776, "Sister Li Keeps a Toy Boy?", is the clearest early answer because it turns Toy Boy's situation into a concrete plot event.
Chapter 813, "She Has a Child?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event.
A practical answer is Chapter 2, because it turns the idea into a concrete scene. The chapter opens with On the top floor of the Financial Building, a gathering of dark clouds..., adds pressure through In my opinion, Mother should kick out this troublemaker who's a stranger., and ends on Jiang.
Chapter 570, "A Simple Auto-Attack Baits Out an Ultimate from the Enemy", is the clearest early answer because the buildup finally cashes out in a real clash.
Chapter 784, "Slander Brother Yang Without Any Proof?!", is the clearest early answer because it turns Mr's situation into a concrete plot event.
Chapter 734, "Are These Primary School Students Arguing?", is the clearest early answer because it turns Brother Jiang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Old Sir Jiang was especially happy.
Chapter 558, "The Love-Struck, Flirty Wife Appears", is the clearest early answer because it turns Luo Xuan's situation into a concrete plot event. The setup is These people's comments are just too unpleasant;