Some people maintain that the whole of medieval society can be explained by the relationship between lord and vassal. But while much of Europe was divided into fiefs, the very lands from which the word feudalism derives, there were parts of the continent that did not fit so nice a definition. In fact, much of Italy, Spain, and southern France were not "feudal" in this sense.
Still, contractual obligations based on land ownership did define much of the social interaction of the period. Corporations, such as they were in this time, might be lords or vassals or both. For example, a monastery might be the lord to the tenant who resides in one of the manor houses on the monastery grounds, while at the same time the monastery pays its homage to the king. There was much room in the medieval system for ambiguities, but the exchange of obligations between superior and inferior was the key element of the society.
As the feudal system increased in scope, new social structures emerged to help maintain the convoluted networks of relationships required by feudalism. In order to justify the continued extraction of resources from the peasants, the kings and lords had to provide security. This exchange led to both the aristocracies that would come to control Europe for centuries to come and the rise of a dedicated warrior class, whose militaristic tendencies would be partly responsible for the years and years of warfare that would wrack Europe. Counterbalancing this, though, was the tradition of courtly behaviour and romantic love that would not have arisen without the fighting class.
While the lives of the vassals were not to be envied, it would be remiss to think that medieval kings led luxurious lives. The great kings of early Europe had more freedom, but that freedom was tempered by the paucity of options available. There was not much more to do than eat, sleep, pray, hunt, and watch over the estate.