Sadness is a negative attribute, the opposite of joy, in which a person feels that his situation is dire as he sinks into a state of unwillingness and inability to do anything.
Hasidism emphasizes that sadness for any reason is a bad trait, like anger or pride.
Even more than the other negative attributes, it is a great obstacle to divine service, for a person who is sad cannot connect with the divine.
Therefore, one must avoid sadness to the best of one’s ability.
–Rabbi Adin Steinsaltz