This is the course of younger sons among almost all Feudal societies in the era. Be it England, France, the Italian Kingdoms, or in Scandinavia.
Younger sons generally did not inherit much land, wealth or power. Therefore a good way for them to gain such was to go off in search of such riches in other lands. It may be in the Crusades, where they went to the Holy Land in search of slaves, gold, titles, or land. Or it may be in fighting against other neighboring kingdoms.
Or as the Vikings where they did not have much wealth to take from each other they went and raided other nations far removed form their lands.
It really is all the same thing. And the Vikings were doing nothing different than younger sons all over Europe were doing at the time. Even the younger sons of Arab nobility did the same damned thing when raiding Europe and Africa.
And also among all three of those major groups, religion was a key part also. In most of Eurasia and Africa, it was Christians against Muslims. The Vikings were the last large group of Pagans in Europe, so they were mostly attacking the Christians for the same reason they were attacking the Muslims.
But there was absolutely nothing special or unique in what the Vikings were doing compared to any of the other groups in that hemisphere at the same time.