The Tanya addresses the issue of sadness and says that there is positive sadness and negative sadness. Positive sadness comes from remorse of our having sinned or having made bad decisions that we might regret. After temporarily getting past this sadness, we are left with a strong resolve to be stronger and wiser in the future. Negative sadness comes mainly from worrying about material things such as wealth, health, and our children. When we worry about any of these items, it is a reflection on our being disconnected from Hashem's love and protection. Is such worries overtake us, we are left with a feeling of lethargy and a lack of enthusiasm in facing the world. Laziness and depression is not far behind. Feeling G-d's love and protection gives us the alacrity and excitement and joy necessary in coping with life in a positive way.