I'm sure this is part of it but I do think it speaks to our society a bit. IMDB released a list of their highest rated quotes from 2000-2009 here:
http://www.imdb.com/list/aS4ZSMt5jtk/
I know it is IMDB and not AFI but I often find my tastes closer to IMDB's top movies than AFI's so I think it is a quality source.
That said, there are a few good quotes from the decade with ones such as "Are you not entertained" and many from the Batman series like "Why so serious", "You either die a hero, or you live long enough to see yourself become the villain." "So we hunt him because he can take it..." are all great quotes but Gladiator and Batman account for many of the actual serious ones.
But you look at the list and while I enjoyed movies like Old School and Anchorman and am part of the younger generation, I'm not sure I'm willing to put "We're going streaking" or "I love lamp" in the same ballpark as "The truth, you can't handle the truth."