One of the central characteristics of chasidic thought was the injunction against being sad.
Unlike bitterness, which can be productive, sadness was regarded as a kind of self-indulgence that drains man of his spiritual resources.
A person who is sad about having sinned, for example, loses the capacity for change and is likely to be caught in vicious circles from which there is no exit:
Sin creates feelings of guilt, which generate sadness and dejection, which lead to despair and to surrender to the urge to sin again.
The chasidic prescription is to strive for a good disposition, to will oneself to be happy even in bad times.
–Rabbi Adin Steinsaltz
From The Tales of Rabbi Nachman by Rabbi Adin Steinsaltz