I also grew up relatively poor (we had food, but couldn't afford e.g. school books) and am currently pretty well off (saving more than half of my salary while enjoying a decent quality of life).
Such an upbringing leaves a mark. The other day I was thinking how it's been going too well for too long and I probably have, I don't know, ball cancer which is going to bring my situation to a more familiar level of misery.
I've lived my life having a plan B, C and often also D for every situation.
When you're not used to the good stuff, you become suspicious to the good stuff once it comes along.
The existing reply captured it well. To add to that, this is not “an unexpected expense”sinking your finances. Imagine instead that you suddenly can’t do the job you do nor can you get equivalent pay. How long could a you float on say minimum wage? Maybe it gets worse and you have trouble getting any regular job. How long could you go with no pay? Many people would say, “can’t happen to me.” And that is what the article is about. That is the invisible knife edge. Get a divorce, get sick, and have student debts, and you could be just like the author and living in a tent and taking years to claw your way out of the situation. When that is a reality that you expect could happen, you think differently. And for most people, it could happen.