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Let's say the average cost of the stock market is $1 for every $2 of expected income. People say "Oh, that's cheap, stocks are a good idea to buy". Everyone starts buying stocks, and the prices rise. Eventually stocks are $1 for every $10 of expected income. People say "Oh, stocks are not going to go up much more, they're expensive. Hrm, Real estate is only $1 for every $2 of expected income. Let's move there". People move there money into Real estate. Or bitcoin. Or Bonds. Etc.