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According to the stereotype, the northerner is utterly straight forward. He is telling everything as it is, straight from the gut. Uncomplicated and unpretentious, he is free of any urbane streaks. And his language of course, is juicy and full of imaginative swearing. An independent soul, never giving up no matter how dark things (literally) look. And as for humor, it is at times dark, at times light hearted, but mostly very coarse and a lot of the time of a sexual nature.
I guess the development of the nature of the northerner (perceived or real) can be at least partly explained historically. (Note: the following is largely speculation and my interpretation of things. I do not have very much academical backing for my claims here.) The northern parts were (and some say it is still) quite provincial, being the back waters of Norway. In fact large parts of the borders were not drawn out until in the mid 18th century, at least in part reflecting the irrelevance of this area for the government in Copenhagen (as Norway was in union with Denmark at this time).
It is easy to imagine how people in this situation would develop a character different from the urban centers down south. This situation would have encouraged a mentality of self-reliance, as people would have to cope with their lives without any help from outside, and in short a "tougher" and coarser mentality than you would typically get in more urbane settings. And of course, as people sometimes tend to define themselves by their differences compared to others, this might have been a self-reinforcing process as well: "We are a tough bunch, much tougher than those sissies down south"
Even today, a lot of northerners are defining themselves based on the perceived differences from southerners. We up north have an image of the "typical" southerner as a wimp and office rat, sipping his caffé latte or whatever they drink down there, and we define ourselves pretty much as the antithesis to this. We take pride in being able to withstand the harsh and cold weather (never mind that there are regions further south which have much colder weather than us), that we have such a coarse and simple humor, etc.