I think the notion of "bubble" simply comes from people's understand that the amount of money poured in for something doesn't amount to very much. For example,

I think the notion of "bubble" simply comes from people's understand that the amount of money poured in for something doesn't amount to very much. For example, say you go to top elite 4-year liberal arts college and receive a degree in gender studies dropping $40k a year. Is that really worth $160k + interest for you? If the perceived value of the degree is substantially lower than expected, you mostly likely wouldn't want to pay back the loan. Consider this sort of scenario for tens of thousands of people. Banks that were expected to get the money back will be left with tons of debters just defaulting on their debt. Principles lost.