I would add to this not selling a position with a loss because the investor doesn't want to recognize the loss. This shouldn't be the primary consideration when making a sell decision. The primary considerations should be the future prospects for the holding and the relative value of the position vs a new holding. If another position provides greater potential - total return, asset allocation, etc, then it may make sense to realize the loss.
Bottom line - investment thesis should carry more weight in the final decision.