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Why is being “upside down” on a mortgage so bad?
The largest problem and source of anxiety / ruin for homeowners during the housing market collapse was caused by the inability to refinance. Many people had bought homes which they were stretched to afford, by using variable-rate mortgages.  These would typically offer a very attractive initial rate, with an annual cap on the potential increase of rate.  Many of these people intended to refinance their variable-rate to a fixed rate once terms were more favorable.  If their house won't appraise for the value needed to obtain a new loan, they are stuck in their current contract with potentially unfavorable rates in the later years (9.9% above prime was not unheard of.) Also, many people, especially those in areas of high inflation in the housing market, used a financial device known as a Balloon Mortgage, which essentially forced you to get a new loan after some number of years (2, 5, 10) when the entire note became due.  Some of those loans offered payments less than Principal + Interest! So, say you move near Los Angeles and can't afford the $1.2M for the 3-bedroom ranch in which you wish to live.  You might work out a deal with your mortgage broker/banker in which you agree contractually to only pay $500/month, with a balloon payment of $1.4M due in 5 years, which seemed like a good deal since you (and everyone else,) actually expect the house to be 'worth' $1.5M in 5 years. This type of thing was done all the time back in the day.  Now, imagine the housing bubble bursts and your $1.2M home is suddenly only valued at, perhaps, $750k.  You still owe $1.4M sometime in the next several years (maybe very soon, depending on timing,) and can only get approved financing for the current $750k value -- so you're basically anticipating becoming homeless and bankrupt within the same year. That is a source of much anxiety about being upside-down on a loan. See this question for an unfortunate example.