Let's think of what's going on with a trader as the stock he bought moves in his favor or against him. When the trade moves against him he hopes that it goes back and makes him whole. In order to trade accordingly to this rule, we have to switch these two emotions. You have to fear when the stock goes against you and cut your losses right away. We have to hope your trade works out OK and stay with it as long as a sell signal is not generated.
Emotions are the greatest enemy of a trader.
A certain level of a trader's development leads to the state when trading decisions are generated and executed with no emotions at all. It is because when we are hanging the picture, we don't feel like our money is on the line. From this, here comes the next switch. It's just not a subject of your job as a trader. Your job is reading stock movement.