For what reason did the Vikings raid and pillage other people?
There are two given reasons as to why the Viking raids of the Middle Ages occurred. One of these reasons is that at the time Scandinavia was experiencing a period of rapid population growth which had led to a shortage of land and resources.
These shortages then forced peoples to travel south in order to raid villages and towns in order to take their goods to survive. Another reason may have been merely that the Viking people saw an opportunity to take the wealth and resources of villages that were undefended and ripe for the taking. The quick hit and run tactics of the Vikings made it impossible for people to anticipate or react to their attacks in time, which led to them being able to pillage villages practically unhindered.
In all, it was likely a combination of these two aspects that led to the rise of the so-called Viking age of raiding and pillaging Northern European towns and villages.