From KeyStone's article: "The Ebb and Flow of the Market"Warren Buffett’s mentor, Benjamin Graham, described it best with his theoretical investor, “Mr. Market”, in his 1947 book, The Intelligent Investor. He says that Mr. Market is an investor driven by panic, euphoria, and apathy, who randomly acts on his mood (Optimism and Pessimism), rather than fundamental analysis. But luckily for prudent investors, it is Mr. Market’s emotional outbursts that can lead to investment opportunities. However, it is uncovering these opportunities when Mr. Market is having an emotional outburst which can be the difficult part. Buying when a stock is undervalued and selling when it is potentially overvalued.
The key is to always know why you own a stock and to not let Mr. Market get the better of you. The market or an individual stock may swing rapidly, if you do not know why you own it and its price begins to fall, you will likely not know what to do about it. Is it time to sell, time to buy more? Surely it is a careful practice, but if time is taken to assess the fundamentals of a business and to interview management - as long as the investment thesis is still intact – Mr. Market is only taking you for a short blip over a much greater ride.