Cognitive dissonance. We're constantly told how there are no substantive differences between us all. Any differences between us are superficial and if these distinctions aren't self expression, which is acceptable, then they are just flawed prejudices in our perceptions of others. Life does not function this way however. So when a child and parent have a relationship, the fact that the roles must differ in order to operate correctly comes into conflict with the egalitarian lore that all the 'good persons' accept.