In shia books
Why Shias mourn?
Its because of the curse of Zainab (may Allah be pleased with her), Sister of Imam Al-Husain, daughter of Imam Ali (as) and Fatima Zahra (as), after the shias' treachery to Hussain (may Allah be pleased with him)
Zainab (may Allah be pleased with her) said:
"O people of Kufa (i.e shias) , May you always shed tears."
When Zainab reached Kufa, she addressed people with fury words: "Praise to Allah, and may the blessing of Allah be upon Muhammad and his progeny. O people of Kufa, you are hypocrites and deceitful. You feign to be sorry for the death of my brother and his companions. May you always shed tears. I find nothing in you but flattery, evil acts and thoughts, pride and spite and ill will. By Allah (SWT)! You deserve lasting sorrow instead of joy. Shames on you, your hands are imbrued with the blood of the son of Prophet Muhammad (saw), the one who was your sole refuge in case of adversity. By your evil act and disloyalty, you incurred the wrath of Allah (SWT) against you. Woe betides you! No one will intercede with Allah (SWT) for you."
As you can see, that Zainab (may Allah be pleased with her) cursed shias that they may shed tears always and they deserve lasting sorrow. Indeed the mourning of shias is due to that curse upon the shias.
Ali (رضّى الله عنه), Hasan (رضّى الله عنه), and Hussain (رضّى الله عنه) hated the Shia
Ali condemned the Shia multiple times, and it is all recorded in the Nahjul Balagha, which the Shia consider sacred. Let us examine Sermon 34 in which Ali (رضّى الله عنه) says that his so-called supporters are not really his supporters and that he wants to be rid of them. You can also view the sermon on Al-Islam.org which is the most reliable Shia wesbite on the net.
“Nahjul Balagha, Sermon 34
During the fight against the people of Syria [Muawiyyah], Ali said to his Shia:
“Woe to you. I am tired of rebuking you. Do you accept this worldly life in place of the next life? Or disgrace in place of dignity? When I invite you to fight your enemy your eyes revolve as though you are in the clutches of death, and in the senselessness of last moments. My pleadings are not understood by you and you remain stunned. It is as though your hearts are affected with madness so that you do not understand. You have lost my confidence for good. Neither are you a support for me to lean upon, nor a means to honour and victory. Your example is that of the camels whose protector has disappeared, so that if they are collected from one side they disperse away from the other side.”
source: https://www.al-islam.org/nahjul/34.htm ”
Hasan did not trust these Shia either as they were very disloyal. In his book al-Ihtijâj, the prominent Shia author Abu Mansur at-Tabarsi has preserved the following remark of Hasan:
“By Allah, I think Muawiyyah would be better for me than these people who claim that they are my Shia.” [Abû Mansûr at-Tabarsî, al-Ihtijâj vol. 2 p. 290-291 , Mu’assasat al-A‘lamî, Beirut 1989]
Distrusting his Shia, Hasan (رضّى الله عنه) made peace with Muawiyyah (رضّى الله عنه) and gave him the Caliphate. His Shia protested at this, and Hasan’s reply (رضّى الله عنه) is preserved in the most important of the Shia books of Hadith, Al-Kafi:
“By Allah, I handed over power to him for no reason other than the fact that I could not find any supporters. Had I found supporters I would have fought him day and night until Allah decides between us. But I know the people of Kufa. I have experience of them. The bad ones of them are no good to me. They have no loyalty, nor any integrity in word or deed. They are in disagreement. They claim that their hearts are with us, but their swords are drawn against us.”
(Al-Kafi, vol. 8, p.288)
Ali (رضّى الله عنه) condemned the Shia so many times in the Nahjul Balagha. Here, Ali (رضّى الله عنه) says that Allah will destroy the Shia of Kufa if they continue to be decietful, and he asks Allah to change his Shia for other supporters, because he distrusts his Shia who are not really his supporters but rather his enemies:
“Nahjul Balagha, Sermon 25
Ali chastises the Shia of Kufa:
“O’ Kufa, if this is your condition that whirlwinds [of deciet] continue blowing through you, then Allah may destroy you…Your disobedience of your Imam in matters of right and their [the Syrian’s] obedience to their leader [Muawiyyah] in matters of wrong, their [the Syrian’s] fulfilment of the trust in favor of their master [Muawiyyah] and your betrayal, their good work in their cities and your mischief. Even if I give you charge of a wooden bowl I fear you would run away with its handle.”
Ali invokes Allah against his Shia:
“O my Allah, they are disgusted of me and I am disgusted of them. They are weary of me and I am weary of them. Change them for me with better ones”
source: https://www.al-islam.org/nahjul/25.htm] ”
In the next sermon, Ali (رضّى الله عنه) asks Allah to curse the looks of his Shia because they betray him in the Battle of Siffin.
“Nahjul Balagha, Sermon 183
Ali says to his Shia at the Battle of Siffin:
“Keep quiet, may Allah make you ugly, O you with broken tooth. Certainly, by Allah, when truth became manifest even then your personality was weak and your voice was loose. But when wrong began to shout loudly you again sprouted up like the horns of a kid.”
source: https://www.al-islam.org/nahjul/183.htm ”
“Nahjul Balagha, Sermon 4
Ali says about his Shia:
“I always apprehended from you consequences of treachery and I had seen you through in the garb of the decietful.”
source: https://www.al-islam.org/nahjul/4.htm ”
And perhaps the best of all to summarize with, Ali (رضّى الله عنه) says that the people have sinned by splitting up into Shias (sects), and that they have left Islam and instead gone back to the pagan Beduin ways. He tells the Shia that they will dishonor Islam by breaking the pledge of brotherhood with the Muslim Ummah and insulting the pioneers of Islam.
“Nahjul Balagha, Sermon 191
Ali says to his Shia:
“You should know that you have again reverted to the position of the [pagan] Bedouin Arabs after immigration to Islam, and have become different Shias after having been once united. You do not possess anything of Islam except its name, and know nothing of belief save its show. You would throw down Islam on its face in order to defame its honor and break its pledge for brotherhood which Allah gave you as a sacred trust on His earth and a source of peace among the people. Be sure that if you incline towards anything other than Islam. the unbelievers will fight you. Then there will be neither Gabriel nor Michael, neither Muhajirun nor Ansar to help you, but only the clashing of swords, till Allah settles the matter for you…You have broken the shackles of Islam, have transgressed its limits, and have destroyed its commands!”
source: https://www.al-islam.org/nahjul/191.htm ”