America's housing crisis (Systems Thinking approach)

For 150 years, banks acted as a balance when people asked if they could afford a house. The same way you go to a doctor when you think you need help with your health, people went to banks asking for help with their finances. In this case, they were asking if they could afford a house.
For 150 years, banks had a vested interest in getting that assessment right - a loss for the owner and a loss for the bank were the same thing, so banks genuinely did their best to only loan to people who could afford it.
One day (though the dismantling of Glass-Steagall, among other things), banks realized that they could package and sell those mortgages to other banks, and other people. They also realized they could get the risk on those packages misassessed, so they got paid more than they were worth, as the risk was underappreciated. This resulted in a new incentive for the bank that bank customers weren't aware of.
For 150 years, banks interests regarding mortgages were aligned with their customers. Suddenly, banks stood to earn a lot more money through selling bad loans than in keeping good ones. So, when people came to ask if they could afford a house, the bank was no longer on their side. Instead, they had an incentive to make bad loans. And they stood to earn more money the worse the loan was.
In short, the system that had been depended on for many years was broken, and only one 1/2 of the borrower-lender pair knew that. The result was entirely predictable.
Had both sides been aware that the bank was aiming to make bad loans, things may (or may not) have turned out differently.

I have heard claims that the people asking for home loans are the ones to blame, because they shouldn't have taken on a responsibility they couldn't afford. But, I think saying that people who have been acting in a stable system are responsible when one party surreptitiously destabilizes it shows a misunderstanding of systems. There was literally NO way for most people requesting home loans to know that their interests and bank interests were no longer aligned. Those people were acting as responsibly as they knew how, and they got fleeced.
If you go to your mechanic, and he tells you that you (for example) need a new cam shaft, but doesn't tell you that he suddenly realized he can make a lot more money on cam shafts than he can on routine maintenance, then how can you make an informed decision, or even be aware of his motivation?
However, I don't think banks are solely responsible, or even majority responsible for the problem. The banks, and the people in them, are actors in a system. Only by changing the system does behavior change (you can see it with the way banks are doing the same things that got them into trouble in the first place, now).
And the only way to change this particular system is with government intervention. Glass-Steagall made the system work in the past, and it can make it work again in the future. Government (and the people that elect that government) are ultimately responsible for the shape of the system, and the regulations that create it. But even then, you haven't found the root of the problem, because Glass-Steagall would be chipped away by officials, as banks ask them to do it, again. History has shown that repeatedly - people forget the lessons of the past, unless they're constantly confronted with them.
Government officials seek reelection. Election campaigns cost a lost of money. It's hard to raise that money from individuals. It's easier to raise it from corporations. Therefore, government officials work to keep corporations happy, since they provide the money necessary to elect them. That means doing what they want, and giving them an ear. Even if they don't talk to the officials directly, money influences people. It creates an inherent conflict of interest to have officials elected by players in a system they are regulating.
If you want to fix the problem, at its root, you have to start with the money in politics.
Force government to provide public funds for all election campaigns, and no private funding, whatsoever. Use whatever criteria are appropriate to select the candidate. Once you have the money out of politics, you can start working on doing what's right for the people, instead of corporations. From that 1 fix, you can start to address the entire system. Until you fix that, you're suturing the skin over an open fracture -- it might make the problem appear to go away, but it only makes things worse in the end.