My friends asked me that approx 1400 years have been passed after the tragedy of Karbala. Still why we Shia's mourn?
Yes imam was martyred very brutally but that incident happened quite long time ago. Can you please help me out with this?

One of the companions of Imam Al-Sādīq (peace be upon him) said to him, “My master, my I be sacrificed for you. When someone loses a loved one, they sit by the deceased, mourning and lamenting. Yet, I see you and your Shī`ā (followers), lament (mātām) and mourn (`azā) from the beginning of the month upon al-Husāyn b. `Ālī (peace be upon him).

Our master Imam Al-Sādīq (peace be upon him), “O you, when the crescent of Muharram arrives, the angels circulate the heavens with the blood stained shirt of Imam al-Husāyn (peace be upon him), a shirt that shows the marks of the swords. We see this, us and our Shī`ā, not with the naked eye, but with our hearts, and upon witnessing us our eyes explode in tears”.

Imam Ridha (as) said:

“The month of Muharram was a month that even those who lived in the era (time) of ignorance used to sanctify. They would forbid fighting in this month. Yet, our blood was made lawful, our sanctity transgressed, our descendants and women were taken captive, our tents were burnt, and all that we owned within the tents were stolen. The sanctity of the Messenger of Allah (peace be upon him & his family) was not reverenced through our matter. The day of Husayn (peace be upon him) has wounded our eyelids, shed our tears, and humiliated (disgraced) our loved ones on the lands of Kārb (adversity) and Balā (affliction), and made us inherit adversity and affliction till the Day of Judgment. Hence, for the likes of Husayn, let the weepers weep, for the tears that are shed eradicates the greatest of sins.”