What are the causes of sadness ?  Losing you . Not been able to know you .  Knowing that I did not appreciate you .  Not been able to experience you and to recognize  that I did not show you the real me .
Sadness is to feel deeply wounded by the person who should had never hurt me. It means losing hope .  Not been able  to see my reflection in your eyes .  

Sadness is not to find me .  Not accepting you, not accepting me, it is to feel resentment, fear, loneliness. It is abandonment, rejection, coldness. It means, not been myself by putting on a mask day by day.

It also means losing my values and my reason, not taking good self care.
Sadness is not taking good care of you. It is to face death, empty handed, not even with a flower.

Sadness is not to know how to share. It is also the result of not been able to learn with due responsibility. It is to act, without thinking of others, but only of me.
Sadness is to bring on sadness. It is to lose opportunities, it is not appreciating, it is to despise others.