After all, if trust is established, you'll be more likely to believe your relationship has a chance of longevity.
Richard Layard, a leading economist, writes in Happiness, Lessons From a New Science, 'As social beings, we want to trust each other.
'In our world the sheer struggle of two people trying to cope with everyday pressures and have a shot at a decent life is immense.
As we live in a time when many relationships seem doomed to end in a dosi-do of partner swapping, few of us hold out much hope of ever celebrating a golden wedding anniversary.
That's why trust is key to the transition from friends to lovers.
Yet not every close friendship will develop into the kind of romance that would give Danielle Steele a run for her money.
'It's the sexual chemistry that ultimately sends you over the edge,' states Christine Northam, a counsellor working for Relate. Each of us has a psychological make-up that has been moulded by life's influences and experiences, and most of the time we're not even aware of it.
Their judgement tends to be more reliable.' Sophie and Simon have now been married seven years and have just had their first child, Maggie Mae. 'I truly believe that because we were friends first our relationship has always functioned on a much deeper level, which is actually quite rare among couples.
It's difficult for me to define why it switched focus that day of the wedding, but, looking back, I think Simon was starting to get under my skin.
Mo Kurimbokus, a relationship counsellor, says, 'Think of it like foreplay.
All the time you're being friends, you're learning about each other.
But, when probed further, one third said they secretly lusted after their male friends.