Some losses do not come back, they become permanent. I have had many individual investments go to zero, bankruptcies, delisted, loans gone bad, fraud, etc. Also losses as a % of total diverisfied portfolio (say 57% peak to trough on S&P) may look tolerable when the portfolio is 1x-5x annual expenses, you are young, working, and accumulating. What if portfolio is 150x annual expenses? a 50% drop means you sit there and see you have lost 75 years of your living expenses. And you need to take xanax and totally cannot sleep.
I have had a lunch once and exited a restaurant to find I had lost 3 years of living expenses on paper during an hour. During the ensuing week I lost another 10 years of living expenses on paper. When I consider I am sacrificing so much to be frugal, working so hard to keep those living expenses down, not buying the pretty BMW, then see a couple of new BMW's being taken in a few hours by volatility (even if it may come back later, a year, many years, after I'm dead who knows), the whole exercise seems too much. Sacrificing so much for what?
I think risk (volatility) needs to be considered not just as % of portfolio, but as % of living expenses, just saying.