What do some stories fall into the Cerebus Syndrome?

I know a similar question had been asked before but why do stories tend to get darker as the series progresses? I know being darker is not a bad thing but I notice some stories that start off lighthearted and silly always become darker and serious as the story progresses. I wonder about that and I wonder if that is a bad thing?

I make a webcomic that starts off lighthearted but has just started to get darker - and yet, that was my plan all along. It's supposed to be like a cartoon TV show, where it's cute and charming, but there's a secret underneath that comes further into light and makes things more complicated.

So sometimes, it might appear as Cerebus Syndrome when it's all planned in advanced? That's from my perspective anyways.

Otherwise... it's sometimes hard to get into a story where everything is so intense all at once. It's why the Hero's Journey begins with the Hero in their "world" as Campbell puts it, and then they experience a call to adventure where they enter a different "world" (figuratively and literally.) And that world is more intense and things get darker. But that's such a meta reason so it might not suit you!

Emotional Torque. To invoke the widest available range of reactions from the target audience. Series generally start lighthearted to be more accessible, it's a bait, to draw people into the series. And then ... BAM!!!

I think it's partially a result of familiarity, too; the writer starts off light, then, as he gets more comfortable in his 'verse, starts to introduce heavier and darker plot and characterization elements.

Stakes dominos too.

Each challenge the main characters must face needs to escalate in order to preserve the feelings of tension. Unless you want to keep beating stronger and stronger monsters until the story runs out of steam, you have to introduce sneakier ways of testing characters mettle, and that road usually runs by betrayal, conspiracy, torture, and unfortunate circumstance.

Now you don't need to go down that road but it's a simple, and as history has shown, effective method of gripping an audience and to a degree, growing with them. Tropes Are Tools after all.

Defying that trend takes more skill and is much riskier so the majority do not do it, and whether that is good or not is probably going to have a different answer from everyone based on taste. I have seen both flavors used to great effect.

If it helps you there ARE stories that start serious and go wacky later on! Like the Anime and Manga City Hunter., topic creator!

It's not inherently a bad thing, no; certainly, it does take some skill to pull off a story with a Cerebus Syndrome, but when pulled off it can serve the story well.

As for the reasons to make it a thing, there are many, both on the Watsonian and on the Doylist end (for those unaware, in-story and out-of-story, respectively). One example would be making a dark story that progresses into a darker one that tries to avoid Darkness-Induced Audience Apathy via giving the viewers and the characters both something lighter to look backwards to and hope forwards to. Another reason would be hitting the characters themselves with a darker-than-previously situation to serve as a trigger for character development.