"Don't let the milestones cause you to miss the simple underlying story: corporate earnings are at all-time highs. So stocks are at all-time highs," says Donald Luskin, chief investment officer at TrendMacro in Chicago. That's how it is supposed to work, he adds.

The S&P 500 — and not the Dow — is also the performance benchmark that money managers are compared against, adds Don Luskin, chief investment officer at TrendMacro. “A pro is only interested in what benchmark he is trying to beat,” Luskin says.

Adds Luskin: “Retail investors who still think about the Dow do so out of nostalgic habit. But investors who make up the vast majority of trading nowadays look at more sophisticated indexes that are better constructed and more diversified.”