It is largely mentality. Both for the initial plan and execution of saving up (and trying to do it fast, e.g. in 5-10 years) and then pulling the trigger and quitting. I pulled the trigger in late 2020 based on somewhat optimistic forecasting (more than the 4%/5% rule of thumb) that I might have enough for it to be indefinite at steady expense levels. TBH rocky markets this year have had me feeling like it'd be nice to have a steady income instead of only outcome (and I should have bought property/gotten a low interest mortgage on something before quitting) but the feeling hasn't been strong enough to make me actually take any steps towards that, and anyway dollar-wise I'm still overall above where I was when I quit. I don't really know if I'll "never work again" (unless forced, and there are several forcing functions I can imagine apart from the market performing much more poorly than expected) but I can easily see myself enjoying the rest of this decade not working, maybe dabbling in some side income gigs eventually, and only getting back into full time employment stuff if I really want to in my 40s. As a man in a knowledge field my "prime earning years" are probably in my 40s-50s anyway, while until aging is solved my physical body's not getting younger.
I wonder what sorts of things influence the mentality. I've been reading Walden here and there recently, I remember being forced to read some of it in I think junior high, I suspect at least a little influence lies there but there's no better trick to make kids dismiss a book's messaging than to force them to read it in chunks via some textbook and then be quizzed. "The mass of men lead lives of quiet desperation." "This spending of the best part of one's life earning money in order to enjoy a questionable liberty during the least valuable part of it, reminds me of the Englishman who went to India to make a fortune first, in order that he might return to England and live the life of a poet." "Yet some, not wise, go to the other side of the globe, to barbarous and unhealthy regions, and devote themselves to trade for ten or twenty years, in order that they may live -- that is, keep comfortably warm -- and die in New England at last. The luxuriously rich are not simply kept comfortably warm, but unnaturally hot; as I implied before, they are cooked, of course à la mode."
Of course by Thoreau's standards I'm positively roasting and happy about it, and I think 10 or even 20 years at a career is more than a fine tradeoff if that's all it takes to secure a retirement. As a teen I remember thinking I'd be happy enough to avoid having worked for 40-ish years straight like most of the adults around me had done/were doing (some of whom died right before or after their retirement age goal, too).