Is it love or chemicals?

So, you fall in love, have high level of "NGF: in your blood, but after a year, the level sinks back to normal. Didn't we know that from all of the great love poetry from the ages? What is the meaning of "Kissin' don't last, cookin' do" except to warn us of this phenomena? Madly in love? It'll pass soon enough | CNET News.com