Many investors will focus obsessively on one investment that is losing money, even if the rest of their portfolio is in the black. This behaviour is called loss aversion.
Investors have been shown to be more likely to sell winning stocks in an effort to "take some profits," while at the same time not wanting to accept defeat in the case of the losers.
"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 since reason." (Philip Fisher, Common Stocks and Uncommon Profits).
Regret also comes into play with loss aversion.
This may lead us to be unable to distinguish between a bad decision and a bad outcome.
"We regret a bad outcome, such as a stretch of weak performance from a given stock, even if we chose the investment for all the right reasons. In this case, regret can lead us to make a bad sell decision, such as selling a solid company at a bottom instead of buying more."
We tend to feel the pain of a loss more strongly than we do the pleasure of a gain.
It is this unwillingness to accept the pain EARLY that might cause us to "ride losers too long" in the vain hope that they'll turn around and won't make us face the consequences of our decisions.