[Quintilian]

Life North of the Wall sucks.

It's cold 90% of the time, there isn't any tillable land, there are Dire Wolves and Giants and a thousand other nightmares. Oh yea, and the White Walkers have come back, and have been slaughtering the population and raising them as zombies.

On the other side of the wall there is a lot of tillable land, and they could be on the other side of a giant fucking wall of ice from all the beforementioned problems.

When the Wall went up the Night's Watch said "If you are on our side, you are a good guy. If you're on the other side, you're a bad guy, and trying to destroy the realm, and thus are Bad."

The whole thing is an allegory for Hadrian's Wall where the Romans built a wall to keep the Scots out, who they deemed as rapers, pillagers and general Gentlemen of Ill Repute. This meant that all the Scots north of the wall had to engage in vicious eternal battle with their most hated of enemies, the Scots.