Is sadness a lack, a feeling of lack of something ... a form of loneliness? something that has been and won't come back? 
In our culture it is not good to show negative emotions, but sadness is part of us, as are joy, or fear and anger. 
In other cultures sadness does not have the same negative value, but it is a bridge to something new. And here the sadness turns into poetry, song, music. 
In Brazil every 30th January the "dia da saudade" is celebrated, it is the presence in the absence, the homeland we left and never comes back, the sweet melancholy that opens up space to desire, to the enthralling rhythm of the samba ...