In general, prison should have five goals, as described by criminologist Bob Cameron: retribution, incapacitation, deterrence, restoration, and rehabilitation. In his words though, "Americans want their prisoners punished first and rehabilitated second."
Norway adopts a less punitive approach than the US and focuses on making sure prisoners don't come back. A 2007 report on recidivism released by the US Department of Justice found that strict incarceration actually increases offender recidivism, while facilities that incorporate "cognitive-behavioral programs rooted in social learning theory" are the most effective at keeping ex-cons out of jail.
The maximum life sentence in Norway shows just how serious the country is about its unique approach. With few exceptions (for genocide and war crimes mostly), judges can only sentence criminals to a maximum of 21 years. At the end of the initial term, however, five-year increments can be added onto to the prisoner's sentence every five years, indefinitely, if the system determines he or she isn't rehabilitated.
That's why Norwegian extremist Anders Behring Breivik, who killed 77 people in a bombing and mass shooting, was only sentenced to 21 years. Most of the outrage and incredulity over that sentence, however, came from the US.
Overall, Norwegians, even some parents who lost children in the attack, seemed satisfied with the sentence, The New York Times reported. Still, Breivik's sentence, as is, put him behind bars for less than 100 days for every life he took, as The Atlantic noted. On the other hand, if the system doesn't determine Breivik "rehabilitated," he could stay in prison forever.
http://www.businessinsider.com/why-norways-prison-system-is-so-successful-2014-12