Political correctness in my opinion is directly related to the idea of using social harmony as a control mechanism. Lets make a few distinctions clear: there is a difference between social harmony and harmony as an ideological ends onto itself. Many who gather under this harmonic ideological template claim to be moderates (truth be told, there are many different types of moderates, this isn't a good type though). I see this trend of political thought to be a full frontal assault on democratic values. The drive for harmony always lends itself to covert forms of censorship, including self-censorship. This tyranny of harmony, when pushed to the extreme, leads to a life of fantasy that shuts out social reality.
This idea of political correctness grew concurrently with the rise of positive or pop psychology, especially in the 1970's and the 1980's. Both concepts are directly linked to each other by virtue of their focus on social harmony above all else. Positive psychology, which you see in a lot of HR training manuals in both the public and private sectors, is a soft version of forced conformity, an attack on the ideas of community and individualism. Psychologists, both inside and outside of government, have learned how to manipulate social behavior. The promotion of collective harmony is just another carefully constructed mechanism for achieving conformity to certain ideals. Positive psychology is all about banishing criticism and molding a given population into a weak and malleable unit that will take orders easily. Personal values, those nurtured by an independent conscience, are gently condemned as antagonistic to harmony. Those who refuse to buy into this broader ideal are ostracized and their opinions are deemed "outside the mainstream". Those who go along are rewarded, depending on whatever field they work in. In the business world, going along might mean pay raises and promotions; in politics, it might mean achieving a higher office.
There is a dark and insidious quality to the ideology promoted by those support positive psychology and political correctness. This form of political thought condemns social critics and iconoclasts, the dissidents and individualists, for failing to surrender and seek fulfillment in the collective lowering of the herd mentality. It strangles creativity and moral autonomy and it seeks to mold and shape humans into a compliant collective. Fulfillment is to be found in complete and total social conformity.
As Genghis pointed out in his comment on this item, this is a form of conformity that's found in all totalitarian and authoritarian structures that seek to dominate. Its false promise of harmony only increases internal anxiety and feelings of inadequacy. The nagging undercurrents of alienation is a trend that's common among most Western nations can be directly connected to this junk ideology that's really just a softer version of totalitarianism.
Positive psychology seeks to create a false sense of enthusiasm, where self-presentation and appearances are valued over authenticity. In this land of happy thoughts and harmony, there are no gross injustices, no abuses of authority, no economic or political systems to challenge, and certainly no reason to complain. That in my view is why political correctness is such an insidious force in the body politic, one that we must battle if we want to maintain a semblance of democratic values.