The notion that home ownership is a priceless boon which could be extended to as many people as possible must be accepted as a significant contributing element to this overall situation. That idea, socially appealing as it is, infected government regulators, economists, credit agencies, potential homeowners, and sinners in general, at ever greater rates over the last ten or fifteen years. A number of related effects then interacted (in a 'boiling frog' way), and collectively resulted in and reinforced overlending and overborrowing:

• A belief among average Americans that home ownership was a quick, and risk-free investment
• Consistently increasing prices in the housing market over a 20 year period, leading to beliefs that 'it will continue forever'. This trend was enhanced by lending agencies incentivized to crank out loan approvals, independent of actual risk, which resulted in a classic economic bubble.
• Federal intervention to keep inflation rates low to spur continued economic growth
• A shared belief on the part of selected government financial leaders that hands-off oversight was the best thing for the economy
• The impact of significant and new foreign capital flowing into US debt markets, further enhancing the availability of cheap money to fund housing
• A global shift towards increasingly exotic (and difficult to understand, monitor, and manage) financial instruments for pricing and distributing risk, and poor transparency regarding the location and extent of those risks
• Recent new standard accounting practices requiring use of mark to market valuation techniques, which exaggerated the impact of volatile property appraisals in a portfolio.
• Rewarding greedy practices within lending firms
• Classical cognitive errors that consistently affect judgements of those who are intended to protect the public from such risks

Collectively, these then resulted in a classic market bubble, not unlike that created during a similar crisis in the 19th century, when railroads, rather than housing, were at the center of problem. That event led to hundreds of banks being closed, four million people losing the jobs, nationwide strikes, and thousands of abandoned Victorian mansions that were left standing empty for a generation, creating an immortal conception we know today as the haunted house. That then led to unmanagable debt, both individually and in governments.

If you want to enable individuals who are inherently greater financial risks to have greater access to home ownership (and thus, to mortgages), then the risks associated with those properties, when aggregated into the broader financial system, will obviously increase the overall risks to that broader system. The system will adjust, one way or another, whether understood or not. You can watch the daily impact summarized here, if you have the stomach it. I'd also suggest reviewing an excellent representation of the interacting factors to this crisis, backed by facts and data. An assessment of which firms lost assets (and how much), and where the money went, also makes for compelling reading. While all these factors interacted,  the idea that housing was a reliable and unconstrained source of wealth available to all, without a counterbalancing expansion of risk that also had to be accounted for, is at the root of this crisis.