Ah, the question of question.
I'll give you the current scientific answer, but then in the next post, we're going to leave that paradigm behind, and dive into the God-based holistic paradigm of mental health, to look at what's REALLY happening at the level of body, mind and soul.
(If you're not into God, then I'll give you some secular sources of help later on, but be warned: secular treatment for BPD has had a lousy track record, and until the arrival of Dialectical Behaviour Therapy (DBT), there wasn't really anything much therapy could do for people diagnosed with BPD. And when it comes to other Cluster 'B' personality disorders like AntiSocial Behaviour Disorder (ASBD) and Narcissism, secular therapy is still stumped about how to tackle the problem. More on this shortly.)
The scientific explanation:
Again in a nutshell, the most recent theory is that people with BPD have a biological issue that causes them to experience heightened fear responses, while at the same time having a reduced ability to regulate their emotional response.
See this article for more details: Grey Matter Changes Linked to Runaway Fear Hub
To put it another way, people diagnosed with BPD have a natural tendency to be more emotionally sensitive than other people, and find it naturally harder to control their emotional responses.
But that's only one part of the equation.
According to Marsha Linehan, the developer of the DBT technique mentioned above, people who have this natural biological tendency to hypersensitivity and extreme emotional responses won't automatically go on to actually develop BPD, unless they're also raised in what she terms an 'emotionally invalidating environment'.
(See this article on DBT, for more details and to hear it from the horse's mouth.)
To cut through the gobbledy-gook, emotionally invalidating environments are when the parents, or main care givers, don't acknowledge and validate the child's feelings and emotions; don't help the child to work their feelings through in a healthy way; don't teach their child the tools they need to manage their emotions properly, and make the child feel like they're 'wrong', 'mistaken' 'incompetent' etc etc, for feeling the way they do, or reacting the way they react.
(And let's remember that these kids ARE much more sensitive than other people, and much more overwhelmed by their emotions to begin with.)
To win acceptance and curry favour, the child tries to suppress, hide and disassociate from their feelings (one extreme…), but can't continue doing this for ever, so they explode out into the open via rage fits, self-harming behaviours, intense 'over sharing', manipulative attempts to keep people close and attentive at all costs, and all the other stuff you just read about (the other extreme…)
If this pattern continues for any length of time, it sets the stage for BPD to flourish, God forbid.
Is BPD hereditary?
Part of the problem is that people with BPD, and other personality-disordered type traits, make very bad parents.
This website, Light's house, excels in painting vivid pictures of what it's like to grow up in a home where one or both of your parents have a personality disorder. It's an excellent, informative resource, but it can make quite harsh, unsympathetic reading for people who suspect they may have BPD (or another personality disorder) themselves! So caveat emptor.
We'll look at the parenting from a slightly different angle when we get to the God-based holistic health stuff a little later on, but let's conclude this section with this:
Studies have shown that BDP might be 'inherited' in nearly 70% of cases.
In the same vein, narcissism appears to be inherited in 64% of cases.
To put this differently, if you have a personality disorder, the chances are very high that it runs in the family, and at least one of your parents also has the same sort of problems.
It doesn't take a genius to figure out that when a parent has a personality disorder, it's very unlikely they'll be able to provide a 'validating' emotional environment for their child.
If a child has a biological tendency towards hypersensitivity and extreme emotional responses (as outlined above), PLUS has personality-disordered parents who raise their family in an emotionally abusive, invalidating environment, the chances of a personality disorder getting passed on to the next generation are much, much higher.
By contrast, when a child with the biological predisposition to BDP is raised in a nurturing environment, the chances that they'll actually go on to develop BDP are much less.