This is one of those topics that everyone likes to have an opinion on, especially smart people :)
Folks that are smart also have a common set of medical conditions. This ranges from everyday annoying like insomnia, NCAH, EDS, IBS, and more to more life long ADHD, ASD, and Alzheimer's. I spent years researching the underlying genetics and how they are connected while working with a doctor to be able to take care of this group better. Being able to help so many fundamentally improve their life has given my life impact in a way that many things haven't. Often small things, but more than we as a society would like to admit, fundamentally changing their life for the better and in the long run helping to dramatically reduce their Alzheimer's risk.
Genetics is a compression system and re-uses stuff. Serotonin in the brain and the gut for example. The fact that we see overlap is both expected, but also a great way to decode what is going on. The fact that I can see their whole genome for under a grand is amazing. Combined with lab work and symptoms treatment has never been easier.
Intelligence can be broken down into a few different things. Starting with raw brain "stuff". Literally needing more brain stuff, faster brain stuff, etc. When I have given talks and walked through all of those mutations I can quickly draw your attention to the common pattern of what they are compensating and I can walk back through human dna several hundred thousand years talking about that, but that simply sets up the hardware. (A big AI model that is barely trained isn't great)
The other half is to keep trying, but trying something different and learn from what it tries. To do this the hormones result in someone who is more on edge, more sensitive to any change, etc. aka they are less happy.
While you can see this individually smart folks seek out each other, have kids and so you see this all together is a bit of a cop out. Long term evolution doesn't care about that. There is a fundamental basic biological systems problem that all of these are compensating for which brings up the how did humans even climb this ladder to begin with.
This heavily overlaps with AI models and at this point I have tools to rapidly convert from the brain and genetics related to intelligence to AI models You want bigger and better brains (more weights, etc) and you want to expose them to more data and you want to fine tune them etc.
A personal favorite topic is white boarding out why brains have ~4.7bit weights and why brains didn't go down to 2bits. I can show this both at the biological level, but quickly go to the information theory math on this.
There is also Williams syndrome aka "happy syndrome". Like anything you want to be able to articulate not just why some folks are sad, but why some are really happy.
This is a deep rabbit hole, didn't even touch on dreaming and many other topics that all influence this in their own way.
So why are smart people not happy? Sure there is always the outliers of bad environment and other causes, but more often than not the story can be found right in their genes and not one gene. Like everything it is more complex than anyone saying it is X.
This is a topic I have fallen into which heavily overlaps with AI for the obvious reasons. If any AI labs want a more detailed Friday afternoon talk on it feel free to reach out.