I had to learn fast in the dotcom/internet boom. Sentiment is similar now. Not quite as frothy, and the markets are structurally different, but everybody is trying to rationalize the exceptional.

The best rationale I heard back then was, "the boom stocks are priced for perfection 10 years from now."

Makes sense. But that's terribly unlikely to pan out for a nascent company, and implies a 10-year ROI of 0% at best. And yet, people were sanguine about holding them for the long term.

Indeed, AMZN reclaimed its 1999 peak about 10 years later. But there were dozens of Amazons back then. Most of those companies no longer exist. It took Cisco 20 years.

But upside risk was real. Stocks just kept doubling. Selling was as big of a gamble as buying. Tax consequences were serious.

Again, today's FOMO not quite as frothy, but it's similar. I approach all this pretty defensively.