Here is some history of the Rangers. First I will lay out the facts and then offer my take...
At the time the U.S. became involved in WWII, most of Europe was engulfed and occupied by Nazi German troops. The First Ranger Battalion was formed at the onset of U.S. involvement in this conflict as an elite unit modeled after the British Commandos. They were highly and rigorously trained to attack in the dead of night from the least likely route.
The Rangers were trained in cliff climbing and speed marches enabling them to infiltrate deep behind enemy lines on foot. Their surprise night attacks often followed 30 mile treks deep into enemy territory. They were also trained in amphibious landings where they would infiltrate from the sea.
This highly trained soldier was strictly volunteer. Each was chosen not only on his merits of physical strength and endurance, but also for his intelligence and ability to think on his feet. The Ranger was expected to act independently and also survive on nothing more than his wits, while accomplishing operations miles behind enemy lines.
http://www.rangerfamily.org/
As I stated earlier the original Rangers were some tough bastards and I have been lucky enough to meet a few. Nowadays Ranger school is seen as a leadership school. No one in the infantry ranks is going to advance without the Ranger tab. Those with the tab are usually farmed out to other US Army outfits. The school ain't that hard. All it takes is determination and luck. When I got out of the service the school was 72 days long. Doesn't really sound like much. And it is not as long as a person is willingly to be cold, hot, wet, hungry and tired for days on end. 
I actually tried out for Special Forces, but I flunked out about 3 weeks into the course...