I know the debate about chemistry rages around here, but A.J. Jacobs - a writer who always inserts himself into his own stories - went a step further. He got an MRI of his brain functions and participated in the same study that Dr. Helen Fisher used to write "Why Him, Why Her?" Fisher was used to studying people MADLY in love - and, in her sample size of 49, saw that the people who were MADLY in love had the same receptors triggered in their brain as people who are doing cocaine.
When Jacobs came in - a 40ish father of three who's been married for 9 years - he was the first subject not to claim to be giddy and weak-kneed in front of his partner. In other words, he was the first normal guy that they studied.
And but sure enough, this very happily married man had it confirmed that while he still is sexually attracted to his wife (according to the MRI), the parts of the brain that are triggered when he thinks about her are the ones that have to do with long-term attachment. Which makes sense.
It's not that it's impossible to be giddy with someone after 9 years; it's that it's extremely rare. And if you spend your whole life chasing that "feeling", you're likely to end up in a series of short-term relationships that end in heartbreak.