It's often said that financial markets are driven by two competing emotions, greed and fear. Skilled investors attempt to control these emotions, and to capitalize on the failure of other investors in that regard. Warren Buffett has repeatedly said that while he focuses on fundamental value, his behaviour is dictated by a simple dictum: "Be fearful when others are greedy, and be greedy when others are fearful."
There's a third emotion that requires constant management: boredom.
It's exciting when assets go up or down by a lot. Generally, they don't. It's boring to watch things that don't do much in a hurry. And it's boring to wait for the market to validate your assessment of fundamental value.
It's boring to sift through financial statements or filings and then discover a company is fairly valued. It's boring to wait for a better opportunity to purchase an asset. It's boring to own a company that has excellent prospects but that no-one has ever heard of (or is likely to ever hear of). It's boring to remain invested in a company that is quietly compounding its value (and whose business you understand well), when new opportunities appear more alluring. It's boring to invest the same way you always have, when the world around is full of "sophisticated" investors raising a lot of money for complex strategies.
Sitting through long periods of boredom is a prerequisite for the fundamental investor. Dealing with this boredom is every bit as important as avoiding being swept away when valuations are high, or being decisive when it seems like businesses, economies or financial markets will never improve.
Pascal said, "All men's miseries derive from not being able to sit in a quiet room alone." I doubt he had investment portfolios in mind, but subduing the third emotion of investing goes a long way to preventing misery of the financial variety.