In nearly all cases this is caused by people living beyond their means - either because of location or other factors. There may be external factors as to "why" they choose to do this:
Family lives in an expensive area and they feel they need to stay close
change in job status and not willing to "downsize" expenses
past poor decisions (credit card debt anyone?)
unlucky events (medical, car issues, etc.)
Some type of addiction issue (alcohol, drugs, etc.)
However, in general most people tend to take the "path of least resistance", even if that path is causing quality of life issues - because change (moving, canceling the $200 cable subscription, not eating out as much) is scary and it's hard. Again, not everyone fits into this category, but, nearly all do - it's common sense.
People feel entitled to a quality of life of what they feel is "right" for them. If their decisions are only impacting themselves and their family, then it doesn't matter as much. If it impacts others (such as having to help someone with taxpayer dollars, charity, etc.), then it's up to those supporting that family (charity, government, etc.) to decide what a "fair" quality of life is.
Most people don't want others starving in the streets, but, at the same time - does that mean we should be paying for free internet and TV? That's up to society to decide what type of world we want to live in, and what is feasible.