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.