When you trade with an amount you can afford to lose, your emotions will always be in check, and as such, your peace is secured, you wouldn't be making outrageous emotional decisions that may warrant you to start losing money.
It's good to think of it before a beginning but it doesn't mean you will feel good when you are facing loss or complete loss of your original trading capital. It's never easy for new traders to afford losing all original capital. This lead to another wrong thinking to "Revenge the market" that results in more losses.
Newbies can start with a good thinking "afford to lose trading capital" but in actual trading, emotion and psychology will be so hard to control and will affect trading decisions. Newbies have to trade a lot to control emotion and psychology, it's only achieveable with time and experience. If you feel it is hard to achieve, just DON't trade.
Calmness is very important in trading, because it will affect the decisions we will take. Once we are affected by emotions, everything will fall apart. It is better when we feel uneasy or start to feel anxious, it is better for us to stop for a moment to restore unstable emotions. Some people experience losses not because they lack knowledge, sometimes they experience big losses because they force themselves to continue trading when emotions are unstable. Although we can be said to be able to recognize emotions, sometimes emotions come suddenly and will be very difficult for us to control, the choice is to stop for a moment.