It's something no-one expects when planning to spend the rest of their life with someone.

Marital breakdown is a fact, though, and while each split is unique one expert has pinpointed what he believes is the number one cause of separation in modern marriages - we expect too much of our partners. Eli Finkel, a professor of social psychology at Northwestern University in the US, has literally written the book on modern relationships - The All-Or-Nothing Marriage: How the Best Marriages Work. He says we need to be more realistic in order to be happy with our other halves. "The main change (in the past century) has been that we’ve added, on top of the expectation that we’re going to love and cherish our spouse, the expectation that our spouse will help us grow, help us become a better version of ourselves, a more authentic version of ourselves," he told The Atlantic. Men and women have shaken off traditional family and gender roles in the last sixty years, he explained, and are seeking to have "more authentic, true-to-the-self" lives. This has led us to want more from the person we spend our lives with, he continued, and is a recipe for disaster when marriages can't live up to expectations. His advice to couples? Ask whether you're asking too much and turn to others in your life for emotional support. "Think about what you’re looking for from this one relationship and decide, are these expectations realistic in light of who I am, who my partner is, what the dynamics that we have together are? "If so, how are we going to achieve all of these things together? Or alternatively, how can we relinquish some of these roles that we play in each others’ lives, and outsource them to, say, another member of your social network?" Eli's theory isn't totally new to us - therapist Shelly Bullard has warned that seeking someone else to 'complete' you can lead to high expectations that will never be met - but it does remind us not to ask too much of our partners. Nagging them about putting a wash on doesn't count, though.