According to Brian Kropp , vice president at research firm Gartner , the average increase in compensation for a worker who quits their old job for a new one is about 15 percent. "You're never going to get that 15 percent [increase] by staying at your current job," he tells CNBC Make It . "That's just not going to happen."

These 3.58 million workers are likely making a smart move, says Andrew Chamberlain , chief economist at job site Glassdoor. "We're seeing high worker confidence in their ability to strike out and find a better job opportunity elsewhere," says Chamberlain. "For many, it's a smart move, as there's a clear advantage to increasing your earning potential by switching jobs."

At the same time, however, the number of workers voluntarily leaving their jobs rose to 3.58 million — that's 2.4 percent of the entire U.S. workforce. Bloomberg reports that the last time this many workers quit their jobs was 2001.

The most recent Job Openings and Labor Turnover Survey (JOLTS) from the Bureau of Labor Statistics (BLS) reports that from June to July, the number of unfilled jobs rose by 117,000, to 6.94 million, confirming the tightness of the current labor market.

The one exception, he notes, is if workers get promoted. But employees now are actually less likely to get promoted internally than they were before the financial crisis.

"One of the big things that happened during the global financial crisis is that organizations pulled out all sorts of layers of middle management, which actually makes it harder to get promoted," says Kropp. "Simply put, there are fewer opportunities to get promoted."

And it's not just antsy millennials who are packing up their cubicles. Some of the biggest increases in quit rates are among older, more experienced, workers.

"Younger employees have always quit at a higher rate. That was true when Gen Xers were in their 20s. That was true when Boomers were in their 20s. That's just a fact," explains Kropp. "What's interesting, is that we are now seeing employees who are more established in their careers also quitting at higher rates."

The incentives to quit apply to workers of all age groups, he says. "The reason people are quitting today is because the labor market is so competitive that the only way they can get a significant increase in income is by quitting and going to another job."

Bestselling management author and CNBC contributor Suzy Welch says that this logic can be simplified further. Welch tells CNBC Make It that before putting in your two-weeks' notice, you should ask yourself, "When was the last time I did something at work for the first time?"

If you haven't been given a new promotion, a new opportunity or a chance to learn something new in recent memory, then it is time to leave.

"The facts are: Be growing, or get going," she says.

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