Neuren Media and Analyst Coverage, page-923

It's loss aversion. People put significantly more weight on a loss compared to an equivalent gain. If a report comes out saying prices are going to fall a bit they tend to freak out and get emotional, whereas a report that says the price is going to $100 would not have an even remotely similar impact. They'd be happy but wouldn't get super emotional and immediately press that buy button. Once investors can identify that common aspect of human nature in their own actions it can make it much easier to avoid it, which can lead to far more profitable trades. It also explains why people tend to sell winners way too early but hold losers forever...