They had a head start after WW 2 for sure. Earlier they were not affected by Ottoman wars like the Balkans and other Eastern Europe (the extent of destruction and de population that brought was beyond imagination), nor a Moorish occupation like in Iberia. Scandinavia was not razed to the ground and drastically depopulated in recent centuries.
Finland is somewhat exceptional among the Nordics as they were quite poor and had some shit to deal with from Russia (and Sweden) but managed to become rich and high trust.
They have been at war for 2 centuries like much of Europe.
Depending on who you ask, Ottomans were their allied or their enemies in Scandinavia, at times they have been both like Denmark allied of Ottomans against Russia that then switched side and sided with Russia that gave them land from Sweden.
There were big differences between Scandinavian countries and they have been at war for a long time.
Denmark for example owned Ghana, like Belgium owned Congo.
They were richer than say Finland.
France has been involved in less wars during the same time span.
Another example: Spain and Portugal weren't bombed during WW2, they were not occupied, they stayed mostly neutral, but didn't develop an high trust society.
Finland is somewhat exceptional among the Nordics as they were quite poor and had some shit to deal with from Russia (and Sweden) but managed to become rich and high trust.