Iceland, Germany, Denmark and Sweden have all very strict naming laws. How did this come to be? Is it a reaction to something in particular? Is there a common thread between the reasons?

In Iceland, the naming laws mainly apply to support the use of patronymics, which are the traditional surnames of Iceland, in the face of growing use of regular surnames, which were mainly Danish in origin. Nationalism was very strong in Iceland in the late 19th century, and after the great lingual "cleansing" that occurred in the early 1800s where Danish influences were purged from the language, it was considered to be logical to extend that to names, as many people had begun using Danish surnames like Jensen or Thorarensen. In 1913, laws were put up that forbade people from taking up surnames although those who already had one could keep theirs, as long as it could be properly conjugated in Icelandic. For this purpose the Mannanafnanefnd, or People's Naming Committee, was put in place to judge whether the names submitted to them were properly Icelandic or not. The laws also forbid an Icelandic citizen from taking up their spouse's surname.

I don't know about the laws of the other Nordics, maybe someone else can help you with that.