Why do we mourn for Al-Hussein (a.s)?

Sadness is a human emotion. A person feels sad when they lose a loved one, whether a relative or a friend. A person also feels sad when he suffers from a certain loss, an illness, or the like. Sadness is thus a natural, instinctive human emotion experienced by every person. We might even say that this emotion exists even in animals, as we observe them grieving when they experience things that occur also to humans, such as the loss of an offspring or similar tragedies.

As for the mourning witnessed during Ashura, it is the grief of a person who empathizes with the tragedies, whether these tragedies occurred in history or are happening in the present. A person who does not grieve for historical tragedies is hard-hearted and cannot grieve for the tragedies that befall people in their own lifetime, such as the savage massacres and the injustices humans inflict upon each other.

There is another specificity to Ashura mourning: we love Imam Al-Hussein (a.s.); because he is the Imam of the Muslims, and because he is the faithful vanguard beloved by Allah and His Messenger(p .). Therefore, when we mourn for Al-Hussein (a.s.), and for the pure elite from his family and companions, we express through this our loyalty to him and our relation with  him; because this connects us with the causes he championed, and with the faith-based and reformist principles he believed in and fought for. 

*From the book "An-Nadwah", vol. 18.