According to a recent Bloomberg article, the fear-of-missing-out (FOMO) factor looms large as the S&P 500 “got off to its best start to a year since 1999,” adding, “It’s risen above worries about North Korea’s ‘Rocket Man’ and the unpredictable U.S. president who revels in provoking him.”

Debating the validity of such fears, the article says, “there’s little debate that there will be something legitimate to miss out on in the stock market in the near term, rather than the hazy distant future.” But the economy, it says, is not fueling the excitement—economists, the article reports, have “gently” boosted GDP estimates, while “equities strategists may find themselves stepping over one another to jack up their forecast of different parameters: the benefits to corporate profits, the subsequent cash returns to shareholders, a return of confidence and greed to the collective investor psyche—and good ol’ FOMO.”

The reality that many investors are coming to grips with, it says, is the prospect of a market “melt-up,” a meltdown that follows a “self-perpetuating” rally and price gains that are “downright ridiculous.” It’s hard to know, the article points out, to what degree tax reform expectations are already baked into share valuations.

“In the near term,” the article states, “there’s a risk that the coming earnings season will result in corporate outlooks that aren’t quite as euphoric as the share-price gains that preceded them.” For now, it adds, FOMO is the “biggest fear investors need to grapple with. That could change quickly, and anyone with the gumption to think they can time the market will need to be on alert.”