I can only speak about my anecdotal experiences. I grew up poor, my parents divorced in middle school putting my mom into welfare status. I worked almost full time during both high school and college to help pay my mom's bills and to buy a car so I could get around. My husband is an immigrant. He also grew up poor. He graduated second in our high school class which provided him with scholarships. He worked part time in college and then full time while going to graduate school. Now, about 6 years after graduating, my husband makes more than $200,000 a year (I am going back to school right now to get a job in the same field as him).
So we have experienced pretty much the entire range of income levels aside from being extremely rich.
The jump from poverty to lower middle class is the biggest jump. I could finally afford to buy a bra that fit me. That was an enormous milestone in my life (I was 22 when I could afford to do this. Cheap stores like Target don't sell my size). I could go grocery shopping without penny pinching or worrying that the debit card would be declined. I could save a tiny bit of money at the end of the month. Something I will never take for granted ever again.
Then the jump from lower middle class to solid middle class was pretty significant as well. That is the first time we started feeling like we had finally become rich (our household income was $45,000 at this time).
Then at one point our joint income was $75,000. We knew we would always have money to pay bills AND we could easily put money away to save for retirement. That felt so odd to us. Nobody we grew up with ever had the money to put it away for retirement. We had to learn a lot about financial strategies. We were never educated in that way. I read several financial blogs to figure it out.
But after that point, it stopped making much of a difference. Keep in mind that we don't have kids yet, so maybe that number would be higher for others, but that feeling that we COULD have kids and get by made such the difference for us. That ability to relax and breathe is something so many people take for granted, especially if they grew up middle class or upper class.
Then my husband moved jobs and started making six figures himself alone. The jump from the $75,000 to six figures didn't change our lives too much. It was more money we were putting into retirement so that's great. But our day to day stress levels hadn't changed.
Now any extra money we make, again, just goes towards our retirements. Our living costs are completely covered. We can financially help out our poorer family members, which is nice. But yeah, at this point, we have learned that happiness doesn't increase anymore. This is when you start realizing that life is empty without deeper meaning. Once your basic needs are met, and you aren't living paycheck to paycheck, and you have an emergency fund in the bank, you psychologically adjust pretty quickly. This effect is called the hedonic treadmill.
Back when I was super poor, I used to think that being a millionaire would make you substantially happier than others. But at this point, I honestly don't think we would be happier if we were to have an income of one million a year instead. I think that we would just psychologically adjust again. Material happiness above your basic needs can only get you so far. It can start feeling empty if you don't search for higher purpose. Giving back to your community, chasing a passion or dream, starting a family, etc. These are the types of things you need to start thinking about because money just isn't going to push you any farther in your sense of well being. That's just what I have gathered from personal experience, anyways.