Why are emotions important when day trading?

Emotions play a critical role when day trading.  It's important to learn how to respond to your emotions.  If a trade works out and you earn and tidy profit, don't get overly excited.  If a trade doesn't work it, don't get too upset.  Doing your best to keep an even keel will help you remain unbiased and follow your own rules.  It ultimately comes down to risk management.  If you respond too harshly to your emotions, you might let a losing trade run too long and lose even more than you intended.  If you let a winning trade run too long you might miss your opportunity to capture profits.  Simply let your trading rules and the charts be your guide.