Most people suspect that something bad is going to happen, particularly when things are going well. About to sell a house? The inspector will find a nest of 6-foot-long termites just before closing. About to cash in your lottery winnings? The ticket will spontaneously combust. Live in the most prosperous nation in history? A large monster will stomp all its cities flat.
Wednesday and Thursday, the Standard and Poor's 500-stock index hit all-time highs. Calm down. It doesn't mean the permanent destruction of your wealth. While there are many valid reasons to sell stocks — which we'll talk about in a bit — a new high in the S&P 500 isn't one of them.
New market highs are unnerving, of course, because the next bear market will start at one of them. You just don't know which one. Had you invested in the market at its all-time high of Oct. 9, 2007, for example, you would have been deeply unhappy. On that day, the S&P 500 hit 1565.15, a level it didn't break again until March 28, 2013. (Investors would have broken even somewhat earlier, thanks to dividends.)
On the other hand, markets generally hit new highs over time, and if you were to sell at each new high, you'd run a decent chance of missing further gains. Had you sold at the market's former all-time high of March 28, 2013, for example, you would have missed a 25% gain.