In the mid-2000s a phrase kept getting banded around in investment management "The search for yield." As interest rates were so low, as were credit spreads, there was a big demand for anything that gave some kind of yield.

That proceeded to drive down credit spreads further.

And more and more.

Which meant basically free capital for investments which were inherently risky, with credit lenders (banks and non-banks) getting money thrown at them for yield, just some yield.

Then they get really clover and making synthetic yield.

And then we had the recent financial crisis, caused by these low rates themselves as a result of the dot-com bubble.

For reading, Shiller's Irrational Exuberance is always fresh (data-based); and stuff by Michael Lewis is good too (narrative).