One reason could be the constant threat to the world in the Dragon Ball universe. She might feel helpless as a non - fighter, seeing her friends and family in danger all the time. For example, when powerful villains like Frieza or Cell were on the loose, she could be depressed about her inability to contribute more directly to the fight.
Relationship issues could also play a part. Maybe she has a falling out with Vegeta. Their relationship is complex, and if they had a major argument or if Vegeta's training and aloofness started to take a toll on her emotionally, it could lead to severe depression. Also, if something happened to her son Trunks, that would be a huge source of grief and depression for her.
There might not be a large number of fanfics specifically centered around Bulma being severely depressed. But it could be that some Dragon Ball fanfics touch on her having difficult times which might include depressive feelings. You could also try reaching out to Dragon Ball fan communities on social media platforms like Reddit's Dragon Ball subreddit and ask if anyone knows of such fanfics.
One reason could be that the fanfic might distort her character. Bulma is a smart scientist, and if the Vegito fanfic makes her act in a way that's not true to her intelligent and rational self, she would seem to reject it. For instance, if the fanfic has her being overly emotional or doing things that are completely against her scientific nature just to fit into a Vegito - centered plot.
One possible reason could be stress. In fanfic scenarios, she might be under a lot of pressure from her scientific research or protecting the Earth. The stress could build up to a point where she starts to harm herself as an outlet.
A young mom might be depressed because she has had to give up a lot for the baby. Maybe she had to quit her job or put her career on hold. This can make her feel like she has lost her purpose outside of being a mom. Social pressure can also play a part. If she feels like she's not measuring up to the 'ideal' mom image that society promotes, she can become depressed. Also, if she has a history of mental health issues in her family, she may be more prone to depression after having a baby.
One reason could be to create a huge plot twist. It's a very unexpected and controversial idea that can immediately grab the reader's attention. Another reason might be to explore different emotional states. For example, showing how Vegeta would react to such a betrayal and how Goku would deal with the moral implications of being involved in such a situation.
One possible reason could be a radical change in Bulma's character. Maybe she's been corrupted by some dark force or has some deep - seated grudge that has been building up over time. But again, this is very uncharacteristic of her normal self.
Well, it could be due to the weight of expectations. Naruto is often seen as the 'chosen one' type of character. In fanfic, Sasuke might feel pressured by this. He might think that no matter what he does, he'll always be in Naruto's shadow. Also, their different upbringings play a role. Naruto grew up alone but with a positive attitude, while Sasuke had a tragic family past. In fanfic, seeing Naruto's strength despite his hardships could make Sasuke question his own strength and lead to depression.
Maybe they're just exploring very dark and edgy ideas without considering the implications. Some people might think it's a way to show a different side of the character, but it's a very wrong approach.
In some fanfics, Bulma might be ignoring Vegeta because she is focused on her own scientific projects. She's always been a brilliant scientist, and if there's a major discovery or problem she's dealing with, she could be so absorbed in it that she has little time for Vegeta. It could also be related to the way Vegeta has behaved. If he's been too overbearing or made some mistakes in their relationship, Bulma might choose to ignore him as a form of punishment or to make him realize his errors.
Well, in fanfiction, one reason could be the loss of his friends. Percy has been through so many battles, and if he loses someone close like Annabeth or Grover, it could send him into a deep depression. Another reason might be the burden of his prophecies. He's constantly expected to save the world, and that kind of pressure can wear a person down over time.