Real love to the Prophet (SAWA) which is supposed to be in the heart of every believer, makes the believers feel sad when remembering sad memories which took place to the Prophet (SAWA and his Holy Progeny Ahlul Bayt (AS). The Prophetic Hadeeth says: No one from you (Muslims) will be a believer, until he feels for me and my progeny more than what feels fo r himself and his family.
Remembering the tragedies of Karbala on Imam Husain (AS) makes the heart of the believer feel deeply sad and grieved. Such deep sadness which is a result of noble feelings, makes the believers shed tears. Such noble feelings do not decrease with time passing but remain alive and warm as far as the love to the Prophet (SAWA) and his Ahlul Bayt (AS) is in the heart.
This is the way of Prophets and their sincere followers as we read in Qur'an. Allah (SWT) ordered the Prophet in Quran to recite on people the details of a tragedy of the pious son of Adam who was killed by his criminal brother. (And recite on them the happenings which took place between the two sons of Adam (Abel and Cain) in truth, when each of them offered a sacrifice, but it was accepted from one of them and not from the other, who said to his brother, I will definitely kill you. His brother replied: Allah accepts only from the pious people, If you extend your hand towards me to kill me, I will never extend my hand towards you to kill you, I fears Allah, The Lord of The Worlds, (Sura 5, verses 27 onwards).
The tragedy of the son of Adam took place thousands of year before the time of the Prophet Muhammad (SAWA), yet, Allah (SWT) ordered the Prophet to recite the story on people, which means that the time passing does not decrease the importance and benefits of remembering it.
Mourning the tragedies of Imam Husain (AS) makes us more near to justice and noble values ordered by Allah (SWT) and more far from Satan and satanic thoughts and acts.
We always need to be more purified to become more near to Imam Husain (AS) and his aims which are the aims of Islam.