There's greed everywhere. I think it's a bit rich to blame Joe the plumber for taking out too big a mortgage when it's offered to him, though.
The charge 'a lack of financial sense' could be levelled at banks offering high LTV mortgages (did they think they boom was going on forever?) to governments unwilling to regulate to slow the boom (some did - in France you can't get a mortgage if the repayments exceed a third of your income) and to financial institutions who can't identify the extent of toxic debt they hold (how stupid is that?) and use financial instruments so complex that very few can understand the debt they hold (also, how stupid is that?).
It's notable that the financially sophisticated are very good at identifying a trend but seem incapable of spotting when a trend is about to reverse, just look at the 'experts' who were forecasting oil at $200 a barrel by Christmas. When the market is going up, oh, it's going to go on forever and when it's going down, oh, it's going down forever. It seems to me a child with a pin could do better.