Saw people bitching on Reddit about how it's implausible for the Primarchs to be so emotionally immature as 200 year old genius war veterans. They even went so far as to cite it as an example of bad writing.
And like...that's wild to me? That anyone thinks emotional maturity is just a thing that happens if you wait long enough? Or that being hyper-intelligent guarantees emotional intelligence? Have you even met people before??
The Primarchs reached physical maturity at a rapid rate, I think the time frame spans anywhere between 1-10 years. So there were Primarchs being treated as infallible demi-god kings after only being alive for three years. They are not having anything CLOSE to a normal childhood.
With normal humans, there's a noted phenomenon where the emotional development of celebrities is often stunted at the age they became famous; if you skyrocketed to stardom at 16 years old, you'll likely be walking around in a 40 year old's body but functioning with a teenagers maturity level. It's because their ability to grow into healthy emotional maturity is disrupted by trauma, a lack of normal social development, intense pressure/responsibility, etc.
Most of the Primarchs grew into 9ft toddlers overnight and then no one had the balls to tell them "I think you're acting like a dumb bitch, actually" ever again. Most of them, from the jump, were taught the entire world revolved around them. That is TERRIBLE for you.
And then there's the fact that a fair share of them had traumatic, abusive childhoods. One of the hallmarks of a traumatized kid is a certain degree of age-inappropriate maturity, like a 12 year old who talks and operates like a grown ass adult. But what people tend to forget is that while traumatized kids are hyper-mature in some areas, they'll also be extremely immature in others.
As a former traumatized child, I had adults remarking on my precociousness all the time when I was a teen; I had a very world weary outlook, I understood and could converse about adult topics, I was extremely self-sufficient...but I'd also have fuckin tantrums over minor inconveniences. I couldn't regulate my emotions even if you'd paid me. I was self-centered and petty and volatile as hell.
The Primarchs who do have some semblance of emotional maturity? Guilliman, Dorn and Khan: three guys with stable, loving family units. Guilliman also had a mom there who'd call him out on his bullshit, he had someone to hold him accountable, he wasn't allowed to be an infallible demi-god. This makes a difference.
And cognitive intelligence really means fuck all when it comes to emotional intelligence. My mom's dad was a mathematics professor at Yale for a bit, but he quit his position because he could not fucking handle students correcting him sometimes. My mom had to grow up in poverty because her dad would have a massive self-destructive crashout and quit a job anytime someone said "Well actually--". He'd grown up being told he was a special infallible genius by a coddling mother and it fuckin destroyed him, it made him insufferable.
Once again: sounds familiar, yeah?
And I shouldn't even have to explain why "if you live long enough, you'll eventually become emotionally mature" is ridiculous, we all know boomers who act like annoying toddlers. That is...insanely common.
Like it really shouldn't be hard to conceptualize why someone who's been denied ANY shot at normal human emotional development would act like a 13 year old boy at 200 years old.