On ABC's Primetime Live, Pitt told Diane Sawyer that tabloid stories saying that his 4-1/2-year marriage to Aniston ended because he wanted children and she did not are, "Ridiculous bull---t ... completely fabricated."
As for what happened, Pitt told W magazine that couples often experience an "unidentifiable malaise." "You don't know what's wrong because the marriage is everything you signed up for."
The end of the wildly biochemical in-love probably feels like "unidentifiable malaise." There are songs about that too, I'm sure ("the thrill is gone" kinds of things) but collecting those would be depressing, so...
ALSO, I thought lately that probably the reason this has been on my mind so much is that I have three teenagers and it might be nice to prepare them somewhat for the in-loves that might come, though I've seen them be very picky about who they'll be interested in. I suspect perhaps it has to do with unschooling, at least insofar as they have been around people of all ages all these years, and they don't have a clique at school expecting them to report all the details of their "dates" or declare who they like.