In medieval Europe, specifically England, there was no concept of social class as we know it today. The population was divided in a sense that could be called instinctive; peasants did not mix socially with nobles because there was a sense that this association would be improper and detrimental to both parties. The peasants would get ideas above their station, and the nobles would lose some of their standing. Each group needed to stay as it was in order to properly function, and each depended on the others to function correctly in order for the social and economic system as a whole to work. Few questioned why different groupings of people should stay apart. In short, it was a system based on caste as opposed to class.
The relationship between castes was somewhat like the relationship between teacher and student today. While the two may interact, one has the role of mentor over the other: one is an authority figure and the other is not. They have different jobs to fulfill--one teaches, the other learns--and both depend on each other in order to fulfill them. The knowledge of this mental ranking usually keeps the two parties from interacting intimately. It would be improper for an equal relationship (friendly or otherwise) to actually occur between a teacher and student; such a relationship is thought to damage each participant as well as the educational system.
Source: my very frightening England 1500-1700 class, plus my head for metaphor.