Sadness is a bad character-trait, which consumes the soul and the flesh, for it comes out from the root of the snake, as the snake’s food is earth, and sadness comes from the foundation of earth. Many evils come from the fact that he serves Hashem with sadness and with a gloomy face. The verse already says “Since you did not serve Hashem you G-d with happiness and goodness of heart from the abundance of everything”. Sadness also causes arguments and quarreling.
Some people become sad because of some bad thing which happened to him whether to his body or his property, and others because they are so arrogant that any time that anybody does anything which he does not agree with or that in his view is not in keeping with his status, they become extremely angry, and when they are so angry but are unable to take any form of revenge, they become upset, to the extent that the sadness is a direct result of arrogance and angriness, and that which comes from something impure, is by definition itself also impure.
And a man who is himself pure should strengthen himself to push away this characteristic from himself by thinking pure thoughts, so that if it came upon him because people did not follow his opinion, he should lower himself and think; “Who am I and what is my life? What will it achieve if someone honors me or denigrates me? A person such as myself is anyway like nothing, and everything is worthless.” If the sadness came because of something which happened, he should think and consider that it is not a coincidence, but it is the decree of the Creator of the world for his good, for Hashem rebukes those who He loves, and he should be happy and consider the honor of the fact that it is clear that Hashem loves him.
However, at the time of the destruction of the holy temple, which was destroyed because of our many sins, then every type of sadness is permitted, and the more a person is unhappy with groans and despondency, with crying and wailing, the more he is praiseworthy. At all other times, however, and especially at times when he is serving Hashem, he should forget his sadness and anger and be only happy and good of heart.