(Note: The author of this fundamental analysis is a financial writer and portfolio manager.)

The market today seems to be more machine than human at times, and the reaction and speed of the market can be unnerving. This continues to show why investing is so much more than just growth rates and earnings. 

Investing is part emotion and part psychology, and understanding that part of the equation is nearly as important as understanding a P/E ratio or a cash flow statement. 

The S&P 500 (SPY) is falling by about 50 bps mid-way through trading Friday, which would seem like just an ordinary day, but it was a decline of nearly 1.7 percent in a matter of only 30 minutes that made the day unique.

The news of former U.S. National Security Adviser Michael Flynn pleading guilty to an FBI charge helped ignite a spark that caused the market sell-off. The sell-off was sharp, and the speed was unnerving. 

Still Fragile

Today's price action shows the stock market is very fragile. And despite the significant gains in 2017, it serves as a reminder that the market can turn at a moment's notice. But one needs to remember what has gotten the S&P 500 Index to the heights seen in 2017.

A Return To Growth

The run in the S&P 500 and stocks in 2017 has been on the prospect of an improving economy, not just in the United States, but abroad. Economic growth, for the most part, has begun to return to the world stage after flirting with recession-like characteristics for most of 2015 and 2016. 

Economic Trends Remain Strong

The U.S. economy has been growing steadily for most of 2017, with the most recent reading of the GDP for the third-quarter pointing to growth of 3.3 percent. Indeed, the Federal Reserve of Atlanta's GDPNow reading points to a fourth-quarter growth rate of 3.5 percent, while signs of growth are returning to Europe. The EUobserver reported that the economy in Europe is growing at the fastest pace in nearly 10 years. 

Corporate tax cuts, infrastructure spending, and deregulation are all things that can help drive further growth but are not the sole reasons for why markets have risen in 2017. 

Earnings and economic growth have fueled the run-up thus far in 2017, and that is what will continue to fuel the markets in the future. Should earnings begin to deteriorate and economic growth fade, then it may be time to be nervous. (See more: Stock Market's 'Absurdly Good' Returns Will Worsen In 2018.)

Michael Kramer is the Founder of Mott Capital Management LLC, a registered investment adviser, and the manager of the company's actively managed, long-only Thematic Growth Portfolio. Kramer typically buys and holds stocks for a duration of three to five years. Click here for Kramer's bio and his portfolio's holdingsInformation presented is for educational purposes only and does not intend to make an offer or solicitation for the sale or purchase of any specific securities, investments, or investment strategies. Investments involve risk and unless otherwise stated, are not guaranteed. Be sure to first consult with a qualified financial adviser and/or tax professional before implementing any strategy discussed herein. Upon request, the advisor will provide a list of all recommendations made during the past twelve months. Past performance is not indicative of future performance.