I Believe In Behavioral Sink, and I also believe it to be the fault of (what else), capitalism. We as a people and a species are increasingly alienated starting from childhood and only increasing in intensity over time. Humans are socially awkward in the same way dogs are socially awkward, and humans are violently self-isolated in the same way dogs can become violently self-isolated. Capitalism views everything outside acquisition of capital or exploitation of labor as an externality, a wasted byproduct to be used if possible. Socialization and the social value of socializing is one such externality.
Reactionaries are so politically powerful with incels because they're right for the wrong reasons when it comes to the sexual pathology the incels obsess over. It is definitely true that society is now socially dysfunctional in a way that it didn't used to be. It's even true that, say, traditional culture forcing women to marry men was a technically more functional and human-oriented facet of society than the machine-oriented alienated society we've got going on now. They've just decided not to come up with any modern human-oriented ideas and instead advocate violently jamming the world and the people in it back inside a box we outgrew because they have just enough knowledge to think because it used to work for different humans it must be valid for us today. Babies, bathwater, all that rot.
Let us consider the social growth of a person in the developed world today compared to the social growth of a person in a society that values socialization, be that preindustrial or Gay Space Communism flavored.
Beginning at birth, the modern child is alienated through the ongoing disruption of their parents. The burdens of daily economic engagement will frequently separate parent and infant, sometimes over long periods of time. We've all heard the stories about children of wealthy parents both modern and premodern who come to see their babysitters as parental figures, but even in lesser forms the infant is often forced into a stack of practical priorities that even with perfectly loving parents will cause them harm. They may be made to wait to eat, may end up breastfed by an unhealthy mother soaked in stress hormones, may be woken out of turn and carted around to loud horrible places then compelled to sleep all while too young to express their feelings towards any of it. They will have long periods of social constriction to only one or two parents that are then unexpectedly expanded and then retracted once again, making them confused about what they are meant to tolerate.
All this, and we're not even out of infancy. This isn't even a toddler yet. And the above is assuming perfectly loving parents with a functional knowledge of childrearing. Basically acceptable parents are going to fuck up a lot more before and after this line.
Alienation becomes a more explicit force to a young child, because they will have begun meaningfully integrating themselves to the fucked-up social system we have and in turn, then become fucked-up. Your own personal fucked-uppedness more likely than not began here even if you don't remember it. The child spends critical years interacting on a daily basis with either just their parents or nobody, in contravention of the longstanding human norm that is communal parenting. This change means that the most primal social instincts of what is normal socialization end up defaulting to near or total solitude. You know how everyone you know flakes out on plans at the slightest opportunity? Yeah. The critical period is not lost but misused.
The child becomes intelligent enough to be taught behaviors and the most basic kind of moral precepts, which is then often framed in a narrative of crime, punishment, and control. Their parents may inflict violence on them in order to enforce behaviors, and the violence is taught alongside the lesson. It is accepted at this basic level that there must be social controls with undesirable consequences placed upon most things. You eat on a schedule. You sleep on a schedule. Time is valuable and must be kept to. Social solutions to problems, already derailed at this stage, are essentially limited to "please just fucking agree not to bite people and keep to that".
The child then enters the school system. The goals placed before them are ones of a rating system. There are particular, mathematically assessed goals they are expected to meet. And yet at the same time these goals are only an approximation, a vague shadow to measure real knowledge and basically nonexistent on the level of practical mastery of a subject. The child learns their basic ideas of worth and quality along the idea of "getting the best score" (and they say you don't play videogames at school).
If the kid is lucky, they'll get to play at recess and sit where they want at lunch. Even then, the vital task of obtaining peers and learning to live alongside other humans is at the bottom of the totem pole and receives only token support from teachers. They might be given a chance to introduce themselves to their classmates, or they might just get thrown into the deep end and asked to swim. That might not be so bad (humans are highly oriented towards socializing, such that we can even socially communicate with other animals) except, remember, they're already a little bit fucked up from being a toddler.
The path to posting incel memes and becoming a mass shooter begins at this moment. If they are able to self-manage their social development from this point onward (be that from unusually good parents, being predisposed to strong socialization, or simple good luck), then they become the kind of person we'd call "confident" or "gregarious". If not, they are likely to enter into various degrees of what we call "social awkwardness", be that simply a withdrawn social attitude or full blown antisocial malignancy. The chances of "falling off the train" are not helped by continuous elimination of social time at school and breaking up connected people by having no space outside classtime and mixing classes at the end of every year.
By the time they are an adolescent, they are "supposed to" (by metric of the hard-won evolutionary niche humans carved out) have become highly socially experienced and able to move onto more consequential interactions, the ones that can potentially result in having children, advancing/regressing in status, or getting your friend killed by a charging boar. But for virtually everyone the "play socializing" period has been warped and corrupted by the tides of our society, and so the more serious interactions have problems as well. It is thus no surprise that the thing the incels obsess over is the first consequential social failure they are likely to experience, the failure to engage in romantic or sexual relationships. It is at this point that most people who have had social problems in their lives realize that they are in deep shit, that they are living in a society full of aliens, and that they don't know what to do (or worse, they think they know exactly what to do and that the Libertarian Party is the only real answer to America's problems).
The failures spiral out from here, and like a toddler who gets angry because they can't speak real words a person who can't engage in serious socialization becomes consumed with frustration. Transition into being an visionless worker drone/useless layabout or disastrously checking out of life in one way or another soon follows.
A final note here: Mass shooters aren't a part of the mental health crisis. The mental health crisis only exists because we're so drastically out of balance with ourselves that damn near everyone is sick, and mass shooters are by and large completely sane. Horribly sane, and consumed by a hatred both internal and external. To dismiss them as "just mentally ill" is no different than dismissing the Nazis as cackling monstrous supervillians when in reality they were what you'd consider "normal people" up until the carefully weighed and accepted decisions of world conquest and genocide.
The world of the alternative is a world that by necessity must reject capitalistic weighing. We can only hope to value our social quality if our ideas of value are not terminally dependent on profit. We will have to "waste money" and we will even have to "waste time". We will have to look at the world in a way that a group of kids sitting around at a park are not viewed as "doing nothing" and we will have to stop spending our whole lives working just to keep the blood money pumping through our bank accounts. We will have to give up on a fantasy of exclusive dominion over our children and we will have to know what our neighbors are like.
We're even going to have to give up dick measuring every aspect of our lives. It's basically that or dystopia.