When a believer realizes the atrocities faced by Imam Husain (as), the leader of the martyrs, and the calamities afflicted on him (as) and his family, he is filled with grief and sorrow. He is also aware that it is Mahdi, the Promised One, who will avenge the blood of Imam Husain (as). Thus, his wisdom exhorts him and his love for the Ahle Bait (as) impels him to pray for the reappearance of Imam az-Zaman (aj), the supplication of a lover inclined to his beloved.
The evidence of this lies in the tradition of Abu Hamza Thumali in the 79th Chapter of Kaamiluz Ziaraat, narrating from Imam Sadiq (as). While discussing the places of these Ziarat, after praying for Imam Husain (as), Imam Sadiq (as) says, “And you send blessings on all the Imams (as) as you send salutations on Husain (as). Then say, ‘O Allah! Complete Your words through them, fulfill Your promise through them..
In another instance, he (as) says, “Then place your head on the grave of Imam Husain (as) and invoke:
اللهم رب الحسين اشف صدر الحسين اللهم رب الحسين اطلب بدم الحسين...
‘O Allah, Lord of Husain, cure the heart of Husain (as). O Allah, Lord of Husain, seek the blood of Husain.’
The reason is quite apparent since it is Imam Mahdi (aj) who will avenge the blood of Imam Husain (as) and cure his heart by taking revenge from his enemies and murderers.