Question: How does one become a “millionaire next door?” What is the mindset by which an individual ends up with a large net worth without a specific goal in mind (unlike the FIRE types, who have very specifically calibrated targets – typically associated with early retirement.)
Answer: It's not really something you achieve as a practical goal, so much as it is a logical consequence of some basic personality traits, such as having a low time preference and high amount of risk aversion. It's taking common sense logic like "have six months of living expenses on hand in case you lose your job" and saying "why not a year? why not two?" It's the belief that sufficient savings on hand gives you, if not FU money, then at the very least "kindly screw off, sir" money. And this isn't theoretical - I really did once resign, on the spot, from a high-paying job where the boss was making me miserable, even though I had nothing else lined up, because I knew I had this sort of buffer. It's reading stories about some family who had some weird medical condition not covered by insurance and instead of coming to the conclusion of "I guess I should vote for Elizabeth Warren" coming to the conclusion of "I guess I need 500K in liquid assets, just in case." It's looking at retirement savings through the lens of "the more I have, the earlier I can" rather than something that happens magically a certain age that Congress decides, or as an overly-ambitious goal that requires huge amounts of sacrifice (like FIRE).
As a practical example, whenever we get some random extra money flowing in, my wife always asks me "What do you want to spend this on?" and my answer is always "Spend? I want to save it!" (I used to say I want to "invest" it but I had to stop because to her "invest" means "gamble and potentially lose it all," even though I've tried to explain that I'm actually much more conservative than the average investor.) I like saving - it makes me feel good. To put it in more concrete terms I try and tell her that every 1K we save earns us 1-2 extra weeks of retirement but it never really connects with her. It's just a certain personality type. At its core, savings is essentially buying yourself more freedom, more optionality. I drive a five year old car that's a little bit beat up and that I don't particularly like. My wife always tells me I should get a new one - we can afford it, I deserve to be comfortable, etc. And she's not wrong - we can afford it. But being able to afford a new car whenever you feel like getting one is a positive benefit in and of itself. And it's the type of benefit that you probably will never experience if you're the type of person who always buys the thing the second you're able to afford it (or even before that). It gives me great comfort to know that if I wake up tomorrow and decide "That's it, I've had it with this car" I could go get pretty much anything I want (within reason) and pay cash and not have to worry much about it. But I only have that comfort because of all of the times I restrained myself from actually doing that.
Being interested in finance/stocks/investments/etc. probably helps support this sort of personality as well. If you're the type of person who only has a basic savings account and an IRA that is wholly invested in a target-year retirement fund (a perfectly valid and defensible strategy!) and all you ever do is shuffle more money into those two things - then savings is boring and you're less inclined to do much of it. But if you're the type of person who is sitting on 4 different mutual funds and a trading portfolio of 10+ individual stocks and some crypto and maybe an investment property or small business or whatever other fun stuff is out there, then deciding what to do and where to put your marginal $1K addition is actually a fun and interesting exercise. You're more engaged and involved with it so it's more meaningful. And if the entertainment value of it all causes you to save 5-10% more than the "everything in the S&P" person does, you're more than making up for the inefficiency of trying to be a stock picker.
As a qualifier, even though I enjoy saving, I still do see it largely as a means to an end, and I don't want to die with millions in the bank. I intend for my savings to help me retire early, not FIRE early, but hopefully at least ~10 years or so earlier than most people (let's say 55 rather than 65). And ideally, assuming my eventual kids aren't hapless losers, the money would basically run out at the end of mine and my wife's life. I'll just probably plan a little more conservatively than most, such that my odds of "leaving a bunch of money behind" will be higher than the typical person - however, their odds of "running out of money to support their desired lifestyle" will be higher than mine!
I'm on board with FIRE. I think it's good to encourage people to save more because the many people seem to consume until they hover near insecurity regardless of their level of wealth. If you look at FIRE forums, the people there are often extreme. They spend a lot of their time trying to optimize their expenditure, so it selects for people who like thinking about this sort of thing. Merely avoiding spending a lot on (1) Housing (2) Car (3) Travel, basically the big stuff does a lot of work. Cutting down on smaller things can be at times more demanding of your attention and time. I think a lot of the people who are extremists on the small stuff actually take some sort of pleasure in it. Which is fine of course. I like getting takeout a lot. Maybe it's not worth it.
I think there are differences in time preference and I think there are differences in early life experiences. I imagine it is difficult to adapt downward harshly if your parents were wealthy. If you were poor then driving a 20 year old vehicle does not seem that hard and living in a small house and not being expensive clothing isn't outside the norm. I think the perfect candidate for FIRE would be a high conscientious and intelligent person who was very poor growing up.
I think being financially secure is a protection against being cancelled. If you are able to save enough money to retire early, then you are able to better speak freely. If you don't have 2 months of savings in your account, you better be careful with your speech.