As with buying at the exact perfect moment, it’s nearly impossible to sell at a stock’s peak. They almost always bounce a little bit higher. By pushing your tracking period out to three months, you avoid killing yourself over a small uptick the day after you sell. Such occurrences are up to the whims of the universe and no amount of studying prices will improve your odds of timing it right. That’s why we look at three-month time frames. You’re interested in trends, not flukes.
―Jason Kelly - The Neatest Little Guide to Stock Market Investing
whims of the universe(宇宙の気まぐれ)ってところがいいですよね。