The rate at which stocks have risen since the Great Recession make 25-year-old Rachel Grafman nervous.

"I don't think the markets can stay this high indefinitely," said Grafman, a civil engineer in Denver. "When we contract, it will be big."

Grafman keeps tabs on domestic equities markets and has some knowledge of market history – yet she admits emotions play into her observations.

Though she doesn't remember the dot-com boom-and-bust, she is aware that it took place and it partly colors her investing decisions.

"I am definitely the more conservative investor among my family and friends," Grafman said. "Most people are more comfortable with risk than I am."

Many people misunderstand risk as a concept, said Priya Malani, a founding partner at financial planning firm Stash Wealth, "not just millennials, but everyone."

After the financial crisis, she said, most people's risk tolerance went down. "And after a nine-year year bull market, that holds even truer," Malani said. "The current market is especially scary for the type of investor who is trying to time their entry or exit point, which studies have proven is impossible to do successfully."

Contrary to popular belief, Malani said, many millennials are saving more than adequately for retirement.

The reasons may be due to concerns over Social Security and the daunting amount of money they'll need for retirement.