Mr. Market is a fitting allegory that Warren Buffett uses to refer to stock market cycles. Think of Mr. Market as your over-emotional business partner; he appears every single day and names a price (stock quote) at which he would either buy your interest or sell you his interest in a business.
Because Mr. Market is an overly-emotional fellow; there are times when he is so overly optimistic that he only sees favorable prospects ahead and is willing to name a price much too high to buy your stake. Conversely, there are times when Mr. Market is so depressed that he sees nothing but trouble ahead and he grudgingly offers a very low price for your interest.
Now we can't control Mr. Market's manic emotional state, what we can control is how and when we act during either his over-euphoria or excessive depression. There is no value trying to figure out why Mr. Market is so emotionally volatile, unfortunately this is exactly what most folks spend their time on.