In ancient philosophy, man is composed of four elements: Water, fire, dust and air. These elements correspond to different middos, emotions and feelings.
The יסוד העפר - dust, is what brings one DOWN. It is the feeling of HEAVINESS, DEPRESSION, SAPPED ENERGY etc. etc.
The causes of sadness are many. One basic cause is the gap between one's expectations and reality.
One important approach is to change one's expecations. Another is to do things they he or she enjoys and not get overwhelmed by negative emotions.