We usually think that marriages have a few years of impassioned romance before settling down into dependable, if less exciting, friendship.
But lots of couples stay in love.
That's what Stony Brook University psychologist Daniel O'Leary found in 2012. He conducted a survey of 274 married individuals in the US asking them to rate how in love they were with their partners.
A full 40% of participants said that they were "very intensely in love."
Similarly, in 2011, Match.com asked 5,200 Americans about how love lasts, and 18% of participants said that they'd experienced romantic love that lasted for over 10 years.
So what gives?
Aaron Ben-Zeev, a philosophy professor at the University of Haifa in Israel and author of "In the Name of Love," writes in Aeon that it's a matter of what you seek in relationships.
Namely, are you after romantic intensity or romantic profundity?
"Romantic intensity expresses the momentary value of acute emotions,"Ben-Zeev says."Romantic profundity embodies frequent acute occurrences of intense love over long periods of time along with life experience that resonates in all dimensions, helping the individuals flourish and thrive."
In other words, romantic intensity is a hit of emotion, which is, of course, fleeting. But profundity is more of a sentiment developed over time. In the same way that seeking out self-realization is a better predictor of long-term happiness than seeking pleasure, pursuing a relationship that's rich with meaning is a better bet than demanding one that's full of thrills.
If you're looking for that profound, long-burning love, Ben-Zeev says to seek out complexity.
"The complexity of the beloved is an important factor in determining whether love will be more or less profound as time goes on," he says.
It works with music: People can enjoy listening to complex songs again and again, but stop enjoying listening to simple ones.
It's the same with love, Ben-Zeev argues:
... a simple psychological object is liked less with exposure, while a complex object is liked more.
A complex psychological personality is more likely to generate profound romantic love in a partner, while even the most intense sexual desire can die away.
Sexual desire is boosted by change and novelty and diluted by familiarity. Romantic profundity increases with familiarity of the other person, and the relationship itself, is multifaceted and complex.
In other words, the more meaningful and rich you can help the relationship to become, the more intensely the love can last.
SEE ALSO: Psychologists Say You Need These 3 Compatibilities To Have A Successful Marriage