Why did the prophet of islam have a beard?

It's fairly clear that the prophet of islam had a beard, and it's also clear that the prophet of islam liked the beard. But did the prophet of islam have a beard because it was a cultural thing at that time and the area he lived, or did he have a beard because he received an order from god that he must keep a beard? I ask because, if it was a cultural thing and not an order from god, doesn't that imply that you should follow the cultural norm of your time and area? In other words, if the prophet had a beard because it was a time/area based trend thing, does that make it a sunnah to follow time/area based trends? Of-course, that would depend on why the prophet had a beard, i.e. an order from god, or cultural/time based reasons. So my question, did the prophet of islam have a beard because it was a cultural thing, the norm where he lived and the norm for the time in which he lived, or did he have a beard because it was an order from god. If it was an order from god, some authentic text references would be appreciated.