Every year, Muslims around the world commemorate the tragedy of Karbala. They attend mourning meetings and processions in which the story of Karbala is retold, lamentations are held and special poetic readers using dramatic techniques and symbolism recite eulogies to mark the events of the day of Ashura. All these commemorative meetings not only serve to convey the events and message of Karbala but also provide opportunities for us to learn about Islam in general.
We commemorate Ashura’ to symbolise and express our grief and emotions towards the martyrdom of Imam al-Hussain (AS), to keep Imam al-Hussain’s (AS) cause alive in our hearts and to be committed to his cause. Also, we want to extract lessons from the tragedy of Karbala and apply them to our daily lives. We want to pay our condolences to the Holy Prophet (S) and the Ahlul Bayt (AS). You might then ask why we do not commemorate the death of the Holy Prophet (S) who has a much higher rank than Imam al-Hussain (AS). The answer to this is that although we do commemorate and mourn the death of the Holy Prophet (S), it is the way and circumstance in which Imam al-Hussain (AS) was martyred which makes all the difference, the fact that he was martyred for refusing a government headed by the wretched Yazid, who drank, gambled and indulged in many corrupt practices, the fact that those who savagely slaughtered Imam al-Hussain (AS) called themselves Muslims is what makes us mourn and lament his martyrdom the way we do.
Imagine, it is afternoon on the day of Ashura’. Imam al-Hussain (AS) is standing on the blood-drenched soil of Karbala in the heat of the scorching sun awaiting to meet his beloved. His head is wounded and blood is streaming slowly down his holy face soaking his blessed beard. He looks around, searching for members of his family and his companions, only to find them brutally slaughtered and slain like sacrificial lambs, lying there on the ground soaked in blood. He hears the innocent crying and wailing of the children and the women. Tears flow down his holy face. He takes a piece of cloth to wipe the blood from his face, whereupon an archer from the army of Yazid (may Allah’s curse be upon him) shoots a triple-headed arrow, which lodges into Imam al-Hussain’s (AS) heart. He pulls out the arrow causing blood to gush out. The Holy Imam is thirsty. He cries: “O Allah, I am thirsty”, in the meantime another man approaches and shoots an arrow which thrusts into Imam al-Hussain’s (AS) throat quenching his thirst. Hilal bin Nafi’ was reported as saying: “By God I never saw a person getting killed with his own blood all over his body in a better state than al-Hussain. I was so overwhelmed by the radiance of his face that I refrained from contemplating killing him.” Even when the accursed Shimr approached Imam al-Hussain (AS) to severe his blessed head, he said: “When I approached Hussain ibn Ali and my eyes fell on him, the light of his face so gripped me that I forgot my intention to kill him.” It is reported that when Imam al-Hussain was killed, the sky rained down blood. It has been recorded that Imam Ja’far as-Sadiq (AS) has said: “…For no one the heavens wept for forty days save Yahya and Hussain…”
Show me such a spectacle of human greatness in an event other than Karbala and I will commemorate its memory instead of Karbala! How can someone hear all this and not cry and mourn; even those who are not Muslims shed tears when they hear this tragedy. Even the heavens wept for Imam al-Hussain (AS).
Let me ask you a question, if a Muslim cries over the passing away of their own relatives, then how can they not cry over the grandson of the Messenger of Allah (S)? As one of the greatest British historians, Edward Gibbon, once said: “In a distant age and climate, the tragic scene of the death of Hussain will awaken the sympathy of the coldest reader.”
Furthermore, crying for Imam al-Hussain (AS) is considered seeking nearness to Allah (SWT) for the tragedy of Karbala is bound to the great sacrifice which Imam al-Hussain (AS) endured for the sake of Allah (SWT) and for Islam. It is only when your heart is soft and absorbable that you feel the sense of closeness to Allah (SWT). The Holy Qur’an praises crying and those who cry for a legitimate cause. The Holy Qur’an describes many of the Prophets and their followers when they cried, “When the verses of the Most Gracious were recited unto them, they fell down prostrating and weeping”
Moreover, we cry over the martyrdom of Imam al-Hussain (AS) for the Holy Prophet (S) wept for Imam al-Hussain (AS) as recorded in many traditions (ahadith): Ahmed and Ibn al-Dhahhak narrated from Ali (AS): “I entered on the prophet (S) and his eyes were flooded, I said: Oh! Prophet of Allah, anyone made you angry? Why are your eyes flooded? He said: Gabriel just left me telling me that al-Hussain will be killed by the river Euphrates. He (the Prophet (S)) said: So he (Gabriel) said: Do you want me to let you smell his dirt (from his burial pot)? I said: Yes! He reached with his hand and grabbed and handful of dirt and gave it to me. So I could not help it and my eyes were flooded.”
Ummul Fadhl the daughter of al-Harith said that she entered on the Messenger of Allah (S) and she said: “Oh! Messenger of Allah, I saw a strange dream last night. He said: And what is it? She said: It was difficult. He said: And what is it? She said: I saw, as if, a piece of your body was severed and was put in my lap! The Messenger of Allah (S) said: You saw well - Fatima will give birth, God willing, a boy so he will be in your lap. Then Fatima gave birth to al-Hussain (AS) and he was in my lap - just as the Messenger of Allah (S) said. So I entered one day on the Messenger of Allah (S) and put him in his lap, but I noticed that the eyes of the Messenger of Allah (S) pouring tears! So I said: Oh! Prophet of Allah, my parents are your ransom, what is with you? He said: Gabriel (AS) came to me and informed me that my nation (ummah) will kill this son of mine.” 
Umm Salamah has said: "al-Hussain entered on the Prophet (S), while I was sitting at the door, so I saw in the hand of the Prophet (S) something he turned over while (Hussain) sleeping on his stomach. I said: Oh messenger of Allah, I looked and saw you turning something over in your hand when the kid was sleeping on your stomach and your tears were pouring? He said: Gabriel came to me with the sand upon which he (Hussain) will be killed. And he informed me that my nation (umma) will kill him."
Ibn Saad, Ali bin Muhammad, Yahya bin Zakariya, a man heard it from 'Amir al-Sha'bi say: “When Ali (as) passed by Karbalaa in his march to Siffien and lined up with Nainawa - a village on the Euphrates - he stopped and called one of them men: Tell aba 'Abdullah (al-Hussain (AS)) what this land is called? He said: Karbala. Then he cried until the earth was wet from his tears. He then said: I entered on the messenger of Allah (S) and he was crying. So I said: What makes you cry?
He said: Gabriel was with me, just now, and informed me: that my son al-Hussain (AS) will be killed at the banks of Furat in a location called Karbala. Then Gabriel grabbed a handful of dirt and let me smell it. So I could not help it, my eyes overflowed.”
From this tradition (hadith) we can see that Imam al-Ali (AS) also wept for Imam al-Hussain (AS). There are more traditions showing that the Holy Prophet (S) wept for Imam al-Hussain (AS).
Umm Salamah, the wife of the Holy Prophet (S) also wept for Imam al-Hussain (AS).
Even the Jinns mourned Imam al-Hussain (AS)'s death, it has been reported that after Imam al-Hussain (AS)'s martyrdom, Umm Salamah said: "I heard the Jinns mourning for al-Hussain (AS).”
In conclusion, we cry for Imam al-Hussain (AS) because so did the Prophet of Islam, Muhammad (S). Every year we continue and revive the message of Karbala by mourning and crying over the injustice which the Ahlul Bayt (AS) suffered on the plains of Karbala, and through this we renew our pledge of allegiance to the Imam (AS) and keep his noble cause alive in our hearts.