Two pages, and no-one's stated the bleedin' obvious yet.
One poster came
this close...
I'm offended by your post, expect to hear from my lawyer. 
...and missed.
The reason for the vast upwelling of political correctness is fear of litigation. Businesses, government bodies, academic institutions and charities
are all vulnerable to lawsuits against them for statements or actions thought to be offensive to certain people, destructive of their dignity and so
on. So these institutions insist on PC, and thus it spreads.
Individuals who espouse political correctness have, however, no such excuse. I take noble, righteous pleasure in offending them.
In America, a nation ruled by lawyers (literally as well as metaphorically, as any rollcall of senators and congressmen will testify), PC will
continue to spread until, presumably, the nation is completely paralyzed and the Chinese or the Russians, who don't go in much for PC, take over.
In fact, PC is mostly an Anglophone thing: Americans are bad enough but Australians and Canadians are worse, and New Zealanders are worst of all.
Notice I don't include Britain itself; despite official efforts, Brits are pretty unPC. As for the French and the Italians and the Japanese and so
on, for the most part they couldn't care less - though Germans and Scandinavians are hot for it. People in my own country don't think about it much,
I'm happy to say, though in not-so-faraway India, where hundreds of millions of English speakers live, it is making steady, slimy progress.
Its origins? Laudable, really. A movement against racial namecalling was probably how it started. The pioneers would have been the best people:
artists, intellectuals, political liberals, scientists and religious leaders of the tolerant sort (as opposed to the commoner intolerant sort). From
such beginnings, it was natural that it should become
de rigeur in academia. In particular, it permeated university arts and social-science
departments. There it fell into the hands of people educated beyond their intelligence (that is what social-science departments at universities are
for) and the rest is history.
Courtesy and nondiscriminatory attitudes are not political correctness, although they are what it originally grew from. Political correctness
-Skyfloating and the rest of you are right - is a monstrous mutant child of the best of parents.