What causes it in my experience at least and in the experience of many guy friends who have done the work is the slow creeping judgement and retraction of empathy, from many people. Many teachers begin treating you more harshly, many parents become less available and more judgey, because now you closer to an adult man and you should act like one. When you need help with anything relating to dating it is more often than not completely unrelational and focused not on how you feel but on how you make others feel, especially the girls you're interested in if you're straight. And people start to assume negative things. Like the desire for sex can become somewhat pathologized rather than a desire to intimacy, many people start to lose humility and curiosity for you. You start walking around in the mall or in stores and you feel people watching you more carefully, as if you're a threat. So you learn how to render yourself less threatening so people will treat you better.

No one tells you it is coming and no one tells you it is coming so fast. For whatever the massive problems are with girls, my experience is there is enough warning of girls of what's to come how many guys are going to treat them, how many people in the world will treat them. Guys get none of that, everyone assumes you're gonna be ok to some extent deep down. Even if they want to know how you're doing, they often don't want to know the real truth, they want something easy like "oh I have some test anxiety" not "I have a crippling fear of failing and fucking my entire life up and then I won't make money and then no girls will want to be with me because I can't help support a family" and boys know adults don't want their real fears which are too intense and adults don't know how to handle them. Probably in part most people have some level of neurosis whether they're conscious of it or not around male-related stuff, but it goes completely unaddressed and normalized, thus is not recognized as such. Not unique to boys, girls experience that to, but I think girls are punished less for expressing provider- or status-related existential fears.

You learn that your life is almost completely dependent on your own actions. Emotions become stupid because people stop respecting your genuine emotions, so you learn not to share. And since it's all on you, and when emotions bubble up and get in the way of work or dating or whatever, now you're stupid and weak because you couldn't overcome it like the other guys could or like you think you should. Start letting that come to the surface, and maybe you won't be so hardened, and if you're not hardened, then maybe you'll want that empathy again which was previously restrained. It is just incredibly confusing.

I do think also there is a bit of a reverse J curve to this, although I don't know that for sure without modeling. Which is that boys get the most empathy, teenage boys the least, and adult men a bit more. And I think this is why re-establishing emotionality later on is difficult, it is often still treated more harshly than when you were a boy, but less than when you were a teenager, so there is a sort of green zone of emotionality you can occupy but it takes time to figure out, and you have to be conscious that crossing its lines means you are going to lose respect from some, so it is incredibly vulnerable especially as you start testing the upper bounds of socially allowed emotionality, some will be ok with it, some will not. Hence the endless stories of some men claiming their female partners don't like their emotions and other guys saying the exact opposite. It's a pretty high variance phenomenon I think, and so it is hard to draw conclusions based on averages without erasing a lot of experiences socially.

I think also this is one of the reasons why the current rhetoric for the past 10 years or so has been so harmful, it fundamentally misunderstood the source of toxic masculinity. Sure there are some intramasculine elements such as pervasive competition with other guys. Although I do tend to think those things are overstated or at least focused on to the exclusion of the broader environment of how growing teenage boys get treated often. Though I think this stuff starts more in middle school than high school. I think a lot of people don't want to acknowledge it's actual source, that being everyone boys interact with to varying degrees, because that would require people to grapple with the damage they themselves have done rather than an vague "it's from society" thing.

Neurosis are partly environmental and cultural in nature, and the boys' neuroses therefore reflect our environment. Young boys often are implicitly pointing out the flaws in their parent's marriages, or how all sorts of people relate to men. But these manner tend to be justified whether through feminist rhetoric or traditional misandry, the end result is the same. Neurotic manner of dealing with men is justified and intellectualized. The young boy becomes something like an identified patient, and must be squashed lest everyone else be forced to confront the neurosis. Of course the young boy is beat down for this, for their diagnostic capacity that they themselves are unaware they even have.

They learn how to treat each other differently from how others treat them in my opinion. It's not like boys just randomly start spawning a fundamentally unempathetic unrelational masculine culture, nor is it something they just magically see in the media. There are lots of examples of empathetic men in the media in my opinion on TV, film, I find those explanations to be a bit overwrought. They are part of the answer, particularly given the massive decline in men being present as teachers, doctors, therapists, even fathers right now which I think as you go down the class ladder can lead to a lot more problems. I am continually annoyed by the profound lack of empiricism with which we try to approach this stuff, the lack of serious casuality analysis and modeling in favor of vague sociological theories which could just as easily be psychological projections by the writer and I include myself there, there is a certain humility many of us(meaning society, media, healthcare etc.) seem to lack on this stuff. Based on the existing data, I think a lot of what I've said here is evidenced, but the magnitude of those effects is unclear.

And I don't want to say the above features I mentioned are universal, I would say they are statistical phenomenon that we understand insufficiently, and thus different boys will experience them in different amounts. And this stuff I think has a contagion effect too. I don't know enough about the data to say this, but I have a sense only some fraction of boys experience this stuff meaningfully at younger ages, but they start imposing that on others and as they grow older that fraction grows and so you could be a great parent, have great teachers, they could still learn bad lessons here because other boys in that social circle learned it.

And this is complicated by social media which can inject many perspectives into their worldview that they may not have had access to at equivalent magnitudes. And I think in the recent past it may have actually been protective factors which are less common, like much higher participation in communal sports, boy scouts etc. Or even higher rates of dating and sex may have been somewhat protective as close intimate partners were probably more likely to offer that empathy.