5. “Diversified” doesn't mean “never going down"
Diversification is not a panacea nor does it prevent your portfolio from falling, even dramatically at times. What it does is protect you from idiosyncratic risk and losing your shirt on a concentrated bet. Buying a car with an airbag is not a bad idea, even if you never get in a wreck. Diversifying your portfolio is similarly wise, even if the benefits may not always be apparent.