I would guess America is so wildly diverse, like most countries, that there’s no real “Average American”. Sure, median income isn’t great and recent inflation has screwed with poorer people all the way through to probably upper middle class.

However, I don’t think there’s “one reason” for people being / feeling poor. It also depends on what you mean by “life changing”.

Anyway - I’d say there’s are very poor people. Some of this is generational poverty, some of it is no family support, some of it is addiction, some of it is mental illness, some of it is bad choices. A social safety net would really help here. For whatever reason the US is against much of one.

There’s working poor, where supposedly the value they bring to a company isn’t sufficient to pay a living wage. I guess elsewhere this is government forced to be a higher minimum wage way more than in the US. This I think is also leading to lots of apathy now in service workers, which means atrocious service at most places, which is finally starting to impact people higher up the income chain. Though it’s usually scoffing that no one is doing a good job anymore.

Middle class people get squeezed by inflation a LOT, and children are incredibly expensive. As you get into middle middle and upper middle classes there’s also the spending problems. This is either driven by lack of or failures of public options, so people are paying for home schooling / private schooling, paying ever more out of pocket to get access to medical care at ever higher rates, and really any service needed can be astronomical or else often worse than doing nothing unless you know a guy. There’s also all the credit card debt that with higher interest rates are squeezing people even more. Some of this is unnecessary spending running out of control, but some of it is lack of wages keeping up with inflation mixed with many things having way more ongoing costs than they used to. Either they have planned obsolesce (Back in the 90s no one had to buy a new landline phone, or wireless phone every 2 years), they have subscriptions (again, in the 90s you bought a car or a radio, you bought a computer, you bought a game, whatever - it lasted with no ongoing payments), and many things just don’t last at all. Then there’s the interest in experiences - which is a transient thing, so you’re always paying for a new one. I’d also say there’s a huge burden on everyone related to college - and it is getting less obvious by the year if that hit in loan payments “forever” is worth the supposed economic boost. My parents grew up when college was like $8,000 max for 4 years and room etc - you could work your way through it, and the boost to lifetime expected earnings was pretty easy to overshadow the effort or loan if any needed. I might have been the last generation to get through with $20k in debt from 4 years, which while I’m still paying on isn’t enough to overshadow the earning benefits. Today (and for the last 15 years or more) it’s more and more for a 4 year degree, while the expected positions aren’t going up in pay very much at all. But if you do go to college that’s going to hit you and your parents for a long time also.

As you get to upper middle and upper class - upper middle is scared shitless by the inflation, wage increase demands, etc that might cause them to fall out of that class, so they can feel “poor” even though they’re far from it.

TL;DR: there are a lot of poor Americans, and many who are “doing OK” either drowning in debt or just feeling insecure even if not technically poor.