So, semi-legendary hero Ragnar Lodbrok who may or may not have existed and may or may not have done the things ascribed to him, supposedly fathered several historically known sons, but before Ragnar, what were the cultural/social/economic factors that led the Scandinavian people into 300 years of raiding and conquest?
There are several factors in play here.
The medieval warm period and land raising (after the ice age) led to higher harvests and a population boom in Scandinavia. There were simply surplus people who could go out and raid.
Scandinavian societies were both "every man is a soldier", since by law (and earlier, tradition) almost all men were free and required to own and train with arms and generally had very even spread wealth (meaning that even the poorest among free men could afford a leather cap, a shield and a good axe). Prestige in viking society laid more in feats of arms and having a large hird (personal guard) that was well-fed and well-equipped than in piety or grand consctruction projects. Compared to the early feudal western Europe, there were a broader base with martial traditions and a larger cultural incentive to go plunder and fight.
The vikings had developed excellent ship building and ship handling skills. Clink built knarrs and longships were veyr seaworthy in the Baltic and North Seas and fast shallow draft ships that allowed the vikings to rapidly sail or row up rivers and shallow bays to attack where attacks were not expected. If encountering resistance, the vikings could just retreat to their ships, sail off and attack a target one or two days sailing away, as they would know that the mass of the enemy forces were still trying to travel overland from where they mast met.
The Franks weakened and then destroyed the Frisian Kingdom/Magna Frisia, which controlled much of the trade in the North Sea and had a strong navy. In this naval power vacuum, the vikings could go trading and eventually raiding with impunity - there were no real navies to speak of in the North Sea and northern Atlantic.
The period also saw decentralisation or even fracturing of lands that the vikings raided. England and Ireland was divided into petty Kingdoms. The King of France often had no authority outside Ile de France, Finland, the Baltic states, Pommerania and Prussia were populated by pagan tribes (some with similar traditions, the Estonians raided Sweden a lot too).
hi! there have been a few similar questions; see here for more responses
Why did the Vikings raid Kells?
Why were Scandinavia and the Steppes so often producing major and disruptive tribes?
What made the Norse/Scandinavians more willing to take the long and dangerous sea voyages they took?