"To me, the answer is simple: the better returns have been, the less likely they are – all other things being equal – to be good in the future. Generally speaking, I view an asset as having a certain quantum of return potential over its lifetime. The foundation for its return comes from its ability to produce cash flow. To that base number we should add further return potential if the asset is undervalued and thus can be expected to appreciate to fair value, and we should reduce our view of its return potential if it is overvalued and thus can be expected to decline to fair value.
So – again all other things being equal – when the yearly return on an asset exceeds the rate at which it produces cash flow (or at which the cash flow grows), the excess of the appreciation over that associated with its cash flow should be viewed as either reducing the amount of its undervaluation (and thus reducing the expectable appreciation) or increasing its overvaluation (and thus increasing the price decline which is likely). The simplest example is a 5% bond. Let’s say a 5% bond at a given price below par has a 7% expected return (or yield to maturity) over its remaining life. If the bond returns 15% in the next twelve months, the expected return over its then-remaining life will be less than 7%. An above-trend year has borrowed from the remaining potential. The math is simplest with bonds (as always), but the principle is the same if you own stocks, companies or income-producing real estate."