This is one of the emotional features among the world of Islam; shedding tears in the face of emotional incidents and phenomena. You see Imam Hussain (as) would cry on several occasions in Karbala. This release of tears is not out of despondency; this is due to very strong emotions, as Islam nourishes emotions in a human.
Imam Hussain (as) began to cry. Then he said—and everyone heard him—“O’God please witness, a young man is going to battle these enemies, whereas he is the most similar to Prophet Muhammad (saw) in appearance, character and thought.”
There is a point I have to make here. Imam Hussain was loved by the Prophet (pbuh) when he was a child. He loved the prophet back so dearly. Imam Hussain (as) was about six or seven years old when the Prophet (pbuh) passed away. The image of the Prophet was in Imam Hussain’s mind as an everlasting memory and his heart held the love of the prophet.
Then God blessed Imam Hussain (as) with Ali Akbar. By the time young Ali Akbar grew to adolescence, he looked exactly like the prophet; it was the face he loved so dearly, he adored so affectionately. Now his son looked so much like his own grandfather. When he spoke his voice resembled the voice of the prophet (pbuh). His character and manners were the most similar to the prophet: equally magnanimous, equally generous and equally gracious.
Then he said, “Whenever we missed the prophet, we would look at Ali Akbar.” So, as this chivalrous young man left to the battle field, Imam Hussain (as) called out, “O Son of Sa’d! May God leave you without progeny as you have left me without.” Hazrat Ali Akbar went to the battle field; he fought courageously, knocking out many enemies; he then returned and said, “I am thirsty”. Then he returned to the battle field.
When he expressed that he was thirsty, Imam Hussain (as) told him: “My dear! Continue to fight for a bit little longer; and it won’t be too long until you are nourished by the hands of your grandfather, Prophet Muhammad (saaw).” When Imam Hussain (as) said this to Ali Akbar, Ali Akbar raised his voice in the last moment of his life and said, “O’ father, farewell! This is my grandfather the Messenger of God who is sending you greetings and calls on to you to join us.”
Ayatollah Khamenei, May 07, 1998