Mr Beets told the NZ Psychological Society's annual conference in Hamilton last weekend that our culture idealised the fusion of two people. The Bible said "And the two shall become one flesh."
But fusion was only a temporary state when two people fell in love.
"We are on drugs: adrenalin, dopamine, serotonin, oxytocin, vasopressin," he said. "The drugs wear off after six to 24 months."
Then two partners typically began to feel constrained by the relationship and wanted to "do their own thing". Mr Beets said people needed to learn how to be true to themselves without cutting themselves off from a partner.
"There are those who respond to the pressure by getting hostile and arguing over little things. But for conflict-avoidant couples, [you coach] them to take risks, to talk about what's going on, to find language that's not aggressive or blaming," he said.
"People need coaching at how to explore conflict constructively. Generally they don't fall apart, because they've been willing to give up quite a lot to keep the relationship going."
He said every relationship was "cross-cultural", even where both partners were from the same ethnic group, because their family backgrounds were always different.
"A very common one is shouting. If you grew up in a very expressive household you can be very loud, whereas your partner who grew up in a quiet household has just not seen aggression. Neither of those people is wrong, but they need to negotiate some acceptable methods for handling their emotions."
He recommends couples set aside technology-free, child-free "couple time" to talk through their differences for 10 to 15 minutes every day, and for a good hour once a week.
