This is the basic wisdom of the stock market: buy a stock when it is undervalued and sell it when it gains in value. This way you maximize your profit on the sale of the stock. Leaving aside the difficulty of predicting whether a stock is going to increase or decrease in value, this action goes against our basic emotional nature. As detailed in the section on behaviorism, we are primed to repeat behaviors that are rewarded and discontinue behaviors that are punished. When a stock is going up, our purchase is rewarded, so we are likely to want to buy. When a stock is going down, we are punished for our purchase, so we are less inclined to buy.