I thought this comment from @once removed deserved it's own thread, as it's a fundamental question I found myself asking during marriage, and still afterward, I am curious:
...
one of the problems with the book (and books like them) is that it takes very basic (if immature) human interactions and describes them in dire terms of a cunning predator mastermind.
there are a few problems with that.
...
This is a good observation of a book that described all negative human behavior as arising from pyschopathy.
I had some concerns when I first found this site, that instead of focusing on better communications with my spouse, I was diagnosing her & labeling her unfairly, and making any sort of resolution impossible.
My XW was never diagnosed w/BPD or any other mental disorder for that matter. But I do know she was seeing a T before we met, and explained the T's comments to her as expressing concern with
her parent's behavior throughout her childhood (frequent fighting, physical & mental abuse, alcoholism, criminal behavior, and mutual infidelity).
Over the years she confided in me some events which certainly would be damaging to any young person, particularly occuring in the formative years, as they did with her. So, while she was never diagnosed, she did have enough

in her past to indicate a basis for distorted thinking, excessive anxiety, and other behavioral issues, if not BPD.
I do wonder - to once removed's point above - if the idea that our partners are BPD stigmatizes run-of-the-mill bad behavior as somehow exceptionally bad & problematic, and makes us less likely to try to resolve long term conflicts and stay together.
I have a friend who's a psychiatrist, and while he agreed it sounded like my wife was BPD, he advised me to try to put concern over that aside, while I tried to resolve conflicts as they came up.
And a MC we had seen earlier in our marriage, later talked to me when we were going through a bad spat, and said he thought my wife was "normal," although some of her behavior was "a little excessive." I told him he didn't know the half of it, and a lot of the time we were in counseling, I was self-censoring because being open
during counseling sessions just lead to a lot more fighting
after the sessions.
In my own experience, I've come to the conclusion that, no, I was not just looking for an easy way out, and I
did try to make it work in good faith; and even though I suspected my XW's complaints were not made in good faith themselves, I treated them so, and only stopped doing that after she was openly inconsistent (showed clear bad faith).
And the issues underlying our long term fights and conflicts weren't the only problem; when I'd keep a journal of the things we fought about, I noticed that she could get heated and hold a multi-day grudge over just about anything, and even months I remembered as being "good" saw us spending 40% of the days either fighting or not speaking to eachother, (sometimes within a day or two of telling me what a good husband I was, and how lucky she was to have me, she'd be accusing me of infidelity because I checked my work email at home, screaming, slamming doors in my face, etc.) And regardless of labels, that's not what I consider a healthy relationship, or one I want to be part of. Also, for the first couple years of our marriage, I didn't even know what BPD was, let alone suspect my XW could have it.
I think, given the length of time a lot of other posters here spent in relationships, it's the same thing in their cases.