My child, always be careful, both when you are happy and when you are sad. When you are happy you should not lose control through vivacity and laughter, nor when you are sad should you be so gloomy that it shows.
The reason is that Satan shoots thoughts at us like arrows, but he cannot tell if our heart is receptive to them. But since he is an expert, once he shoots an arrow—that is, once he assaults us with an evil thought—he observes our face and all the movements of our body, and based on them, he gauges how well the arrow struck our heart. If he sees that the soul is hit, then he shoots more arrows there to kill it. But if he deduces from the exterior signs that the soul is not wounded, he changes his attack, and so on.
Therefore, when you are happy, you should conceal it within yourself, so that he does not discover it through your carelessness and rob you of it with some temptation.
Likewise, when you are sad, do not let it show lest he augments your grief after discerning the reason for your sadness. If you maintain a balanced state when you are happy and when you are sad, the devil does not know exactly what is happening inside you, and thus does not know how to fight you.
"Counsels from the Holy Mountain", Elder Ephraim of Arizona