Financial markets have no biological reality to tie them down After years of reading various things trying to use past financial data to predict the next depression, I read a thing where a guy said that people get lazy and self indulgent during good times and then work harder during bad times and that explained the ups and downs. I'm sure it's more complicated than that, but I stopped trying to find a model that used…
Rather than Lazy/Work hard it might be more speculate/save. In a bull market you are trying to throw more money in. In a bear you are trying to save. It is self reinforcing too.
There's no certain number of years or something that causes it. It's human behavior and real world limits, like Peak Oil.