Sometimes we are our own worst enemies. Research shows that our brains are wired to trade stocks actively, and this often works against us. Even professional managers (such as mutual funds) have a hard time generating consistent outperformance. So, the best managers may be those that understand the psychology of investing and are able to control themselves. See the article here, Wall Street Journal. A good book on the topic is Psychology of Investing, by John Nofsinger.
Dolvin, Steven D., "Control Yourself!" (2012). All Chapters. 29.