...there is a complicating factor that makes the handling of investment mistakes more difficult. This is the ego in each of us. None of us likes to admit to himself that he has been wrong. If we have made a mistake in buying a stock but can sell the stock at a small profit, we have somehow lost any sense of having been foolish. On the other hand, if we sell at a small loss we are quite unhappy about the whole matter. This reaction, while completely natural and normal, is probably one of the most dangerous in which we can indulge ourselves in the entire investment process. More money has probably been lost by investors holding a stock they really did not want until they could “at least come out even” than from any other single reason. If to these actual losses are added the profits that might have been made through the proper reinvestment of these funds if such reinvestment had been made when the mistake was first realized, the cost of self-indulgence becomes truly tremendous...................
Related excerpt from Tren Griffin in Charlie Munger: The Complete Investor:
The deprival super-reaction tendency is more commonly called loss aversion, and it can cause investors to irrationally avoid risk when they face potential for gain, but irrationally seek risk when there is a potential for loss. In other words, people tend to be too conservative in seeking gains and too aggressive in seeking to avoid losses. The most important point to remember about this tendency is that it causes investors to do things like sell stocks too early and hold on to them for too long. It is very common for investors to hold on to losing stocks in the hope that somehow the price will rise and they will somehow break even.