weeping from shia point of view

1) Imām Ja'far  Sādiq (A.S.) said:

"One who weeps for Imām Husain (a.s.) when he is mentioned before him will be rewarded by Allāh Himself, even if their tears are as small as the wing of a fly. And Allāh will not be pleased with any reward for them lesser than Paradise."

 [Reference: Kamiluz Ziyaraat, Chapter 32, Tradition 3, page 102]

Weeping on Imam Husain(AS) from sunni point of view

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.”

Ref : The Decline and Fall of the Roman Empire, Edward Gibbon (1911), v. 5, pp. 391-2.