The Shi'iah and the Sunni schools of thought both agree through general consensus that shaving ones beard is unlawful, because of "consensus" being one of the four sources by which a jurist may give a ruling on a particular issue, it is considered sufficient for use in proving an obscure / unknown issue if an issue can not be solved by the other three sources, but a major difference being, that when the Shi'ite jurist uses "consensus" as a source, his result has to be obtained by consensus but with at least one narration of the Infallibles (as) on that particular issue the jurist is trying to prove, and if that is not possible the jurist would turn to another source to provide an answer regarding the issue at hand .al-Bahai, al-Damaad, and Kashif al-Ghita have given rulings of shaving being unlawful using the above method, in their books al-E'eteqadaat, and Resalat al-Share al-Muqadas

The first cause:
The beard was created for the males when human beings first came into existence, the beard playing an important role in differentiating between male and female, Imam al-Sadiq (as) has narrated:

From the laws of the Lord of the universe was that he gave the male gender of the human a beard so there be a difference between them

Another narration of al-Sadiq (as) states:

If the hair were not to grow at a specific given time, would not (the male) remain in a state like that of a youth and a female, whereas you would not see in him respect and awe

The second cause:

Ibn Sina in his book, al-Qanun,  states:

"Surely the benefits of specifying the beard with the males and not the females is the introduction of beauty, awe and respect which the male requires rather than the female," the above statement confirms the discussion on Prophet Adam (as) and his supplication to Allah (swt), we also observe in the Arabic language the beard having different names, such as; al-Karimah(the respected), and al Mahasin (the beautifier) So logically, who would deny beauty if it were to be offered to him? And who would be prepared to impersonate a female?  And finally, who would prefer to forget his original creation? (i.e. where he would see his beard grow as an adornment given to him by his Lord and Master)