Why Imam Ali (peace be upon him) had tears in the twelfth of the month of Ramadan?

Many historical events showed the great position and status, given to the Imam Ali (peace be upon him) by The Almighty God and His Holy Messenger (prayers and Allah's peace be upon him and his holy Household), which some try deliberately to hide or mislead people on them either by hate or envy as they followed the roads of Satan, {They want to extinguish the light of Allah with their mouths, but Allah refuses except to perfect His light, although the disbelievers dislike it.} (9:32). Among those important events in the life of the Commander of the Faithful Imam Ali (peace be upon him) is the brotherhood established by the Holy Prophet (prayers and Allah's peace be upon him and his holy Household) between Imam Ali (peace be upon him) to himself, which was on the twelfth of the holy month of Ramadan. Noting that the project of brotherhood established by the Holy prophet (prayers and Allah's peace be upon him and his holy Household) between muslims was one of the most powerful Islamic policies induced by the Quran: {The believers are but brothers} (49:10).
The Messenger of Allah (prayers and Allah's peace be upon him and his holy Household) launched since his arriving to the Medina the foundation of the Islamic nation, so first he built the Prophet's Mosque to be a platform for leadership and a center for State-building along with its worship and intellectual duties. Then, he (prayers and Allah's peace be upon him and his holy Household) started constructing the home front and strengthening the social structure, so in the twelfth of the holy month of Ramadan in the first year of the blessed Prophetic immigration, the Holy prophet (prayers and Allah's peace be upon him and his holy Household) established brotherhood between migrants and the Ansar [residents of Medina who supported him].
When the holy Prophet (prayers and Allah's peace be upon him and his holy Household) made his companions brothers, Imam Ali (peace be upon him) came to him with tears in his eyes and said: "Oh Messenger of Allah, You have established brotherhood amongst your companions but have not made me the brother of anyone!" Thereupon the Prophet (prayers and Allah's peace be upon him and his holy Household) replied: "You are my brother in this world and in the Hereafter."
Another narration has quoted this incident in a more comprehensive manner and says that the Prophet (prayers and Allah's peace be upon him and his holy Household) replied to Imam Ali (peace be upon him): "By the Almighty (who has appointed me to guide the people) I postponed the question of your brotherhood for the reason that I desired to become your brother when brotherhood among all others had been completed. Your position vis-a-vis myself is similar to that of Harun and Musa, except that there will be-no Prophet after me. You are my brother and my successor".