This site has answer to something that has long puzzled me. I had supposed that it was over-population that led the Norsemen to invade and keep so many different places.
Here it is said that King Harold wanted a beautiful girl to become his mistress (leman). She declined, taunting him that he was only one king among many.Instead of being angry, he thanked her for having shown him the course he should take. He conquered all the Scandinavian countries, and imposed permanent obligatory loyalty, and taxes. The very large number of Norsemen unwilling to accept this fled and conqured new kingdoms.