It seems to me that most ancient cultures valued beards highly, associating them with virility and maturity, while the Romans associated them with decadent "eastern" cultures and considered them almost effeminate, in a paradoxical way. I do know that letting the beard grow was part of their mourning rituals.
Is there any consensus among historians on why Romans rejected the beard, or any contemporary opinions on the origin of their shaving customs?
That is not exactly true. According to Pliny, first barber came in to Rome during Punic wars, so early republic was obviously bearded. The popularization of shaving is attributed to Scipio Africanus and soon it become a distinction of a true Roman- Easterners and Greeks were bearded, barbarians had mustaches, Romans were clean sheaved.
First shaving even become sort of act of passage in to adulthood (Petronius even mocks over-emphasation of the event in Satyricon- Trimalchio holds his first cuttings in golden box).
Beards regained popularity under Hadrian (again- as a sign of Greek-appreciation) and remained popular- at least among emperors- until Constantine the Great.
In between Scipio and Hadrian beards were grown only by devoted enthusiasts of Hellenic culture
Is it significant Hadrian had his unique type of facial hair?