I grew up in a middle class family by national standards, but we lived in a fairly rural area, so my family was rich by comparison with many of the kids I grew up with. I started dating one of my life-long best friends during the summer before senior year of high school. Her family was fairly poor by my town's standards, and her parents had fairly serious financial problems that directly affected her growing up. She also had to work quite a bit throughout high school.

I never noticed much of a difference between us for the first year of our relationship. Almost every money-spending opportunity available to us was fairly inexpensive (IE almost all the restraints around us were fast food or cheap diners; the largest shopping destination in 30 miles was a walmart; etc.)

However, when we moved to college together (not our plan, but largely a coincidence), things changed quite drastically. She was mainly on private student loans because maximum in federal Pell grants and federal loans was less than half of the cost of attendance. Conversely, my parents paid for everything related to college, except for my basic incidental expenses like food and alcohol, which I paid for. Once school started and she received her loan refund (about 4 thousand per semester - she totally over borrowed for school), she went fairly crazy spending money on useless things. I suspect a lot of her spending habits at the time directly came from the fact that she had never had any real kind of disposable income before receiving her first load refund. (Which is not to say that people who grew up poor don't or can't have good spending habits, but I think this was certainly a factor in her case. Source: she told me.)

I was fairly serious about our relationship at this point, and I was considering getting married after college was over. So, I thought that her spending decisions and future debt load were directly my concern. In retrospect, I realize that this was really condescending, but I often voiced my opinion that she should be more careful with her spending considering it was almost entirely financed by really expensive student loans. This was an almost constant source of tension between us, and it led to some of our most intense arguments. We eventually broke up after college, and one of the few things she said to me in our last argument was that I "would never understand" what it was like to be in her situation because my parents were fairly well-off. I'm not sure that she was right, but our different backgrounds were definitely a factor in the eventual demise of our relationship (in addition a huge variety of other dysfunctions).