...that we love something else. Whaa? Let me explain. Any two people can love each other. Loving another person is quite easy with the attraction, admiration, lusting after, respect (rarely though) and what have you doing bulk of the leg work. Most of the time, however, we measure love with how much of that attraction remains for the other - cool, if that floats your parade balloons.
What's the biggest cause of break-ups? Someone tunes out. Because, after all, loving someone can become boring too when (s)he falls in a pattern. Also, half a couple growing as an individual will cause the couple to limp on till death does it apart. How many times have you heard people say: "My life is a blank canvas now", "I need to find out who I am", "I need to figure out what else is out there"? All are basically a nicer way of saying, "You got boring, amigo/a."
If I'm passionate about making movies and my girlfriend/wife is passionate about genetic engineering, we both have a world we can go to for individual growth and stimulation. We'd then come back to each other a day older and a lesson wiser. We'd share stories of our world away from our world together. I'd love that when we have coffee together in the morning. I'd love that when we meet for lunch. I'd love that at the dinner table or at the couch, watching TV. I'd love that before we turn the lights out and go to sleep - or not ;)
Growth of both partners outside of a relationship is crucial for couple-happiness. Or am I just speaking for myself?
In reality, most couples are just fine giving 'growth' a middle finger and being able to just fuck, eat, and go out. When they start to bore each other after a while, only one of those activities survives. The smart thing to do at that point is to surrender to the half that does the grocery shopping.