Libby Purves: The tears I'll be shedding on Valentine's Day

In the innocence of my twenties I laughed at the Valentine messages from Hugga Bear, Hunky Monkey and the rest. On Tuesday I'll just snivel.

It's like crying at weddings. The young never understand why women over 50 dive for the Kleenex when the bride floats up the aisle. They think it must be disappointment or envy. It isn't. It's experience. We know too much about the bumpy road ahead, the pitfalls, the briars. We cry because they are so hopeful, and if we are reasonably happy ourselves, we wish them to reach that plateau too, and hope they recognise it when they get there.

But you'd be a fool to bet on it. Life is long, women are independent, expectations are high. Divorces end two in five UK marriages, nearly three-quarters being initiated by the wife. Cohabitation has only a 4 percent chance of lasting 10 years. A quarter of British children see their parents split. Divorce doesn't suit children: at best it is disruptive and saddening, at worst catastrophic. The next film to wring our withers is The Squid and the Whale. Based on the childhood of its creator, Noah Baumbach, it shows with rare and raw explicitness how boys of 12 and 16 suffer when their parents divorce. Sounds like a grand night out.

The course of true love never did run smooth: but nowadays the smooth bit comes first. Think of almost any classic love story from Romeo and Juliet on, and the lovers are beset by outside pressures. There are class or race divisions, social disapproval, religion, family feuds, war, poverty. In Jane Austen, the lovers' personal misunderstandings are generally compounded by social or financial pressure; their resolution involves a practical improvement like Captain Wentworth making his fortune at sea or some clerical Edward or Edmund getting a living. Thus when we reach the happy marital ending we tend to believe in it. They've come through a lot.

Today, however, almost nothing external is allowed to separate real or fictional Western lovers. Anyone can date anyone, even if they're married to someone else or of the same gender. Even Brokeback Mountain had to be set 40 years ago in Wyoming in order to make us believe that the cowboy lovers couldn't just make a life together.

Modern lovers stroll in a leisurely fashion towards marriage or cohabitation or just bed, and only then hit turbulence. Films and novels are now about people managing to make themselves miserable without external help.

It may be men who "won't commit" or women who "want more" . It may be disillusion with humdrum life after a starry wedding. How much, I wonder, does the divorce rate tie up with the fact that UK weddings cost, on average, £12,000, including a £3,000 honeymoon? It may be sexual boredom, casual infidelity or the shocking discovery that babies are hard work. Often the problem seems to be competition for status, or inequality of professional success.

No wonder the wartime generation throws up its hands in despair at the selfishness, the materialism, the childishness of it all. If you started out with domineering parents, five years of bomb-scarred separation, two nights in Margate and a few sticks of Utility furniture, it must be hard to see why sleek young newly-weds back from the Maldives can't seem to rub along.

Above it all hover two absurd illusions: the old romantic nonsense that love conquers all, and the new romantic nonsense that a man and a woman can be equal in everything and yet utterly sufficient to one another's every need, never having to say sorry.

But again, fiction comes to our aid with fables for our own time. A slew of chick-flicks offers women the escape clause of female bonding (both In Her Shoes and Rumour Has It... offer the message that men are all very well, but what a girl most needs is a hug with her sister and a tough granny played by Shirley MacLaine). Friends offers the puppyish security of a gang. Bridget Jones peddles the delusion that selfish, idle ignorance makes you so sweet you'll attract a handsome lawyer rich enough to keep you in Agas and girls' lunches.

It's a wonder that even three in five marriages turn out just fine.

Libby Purves's latest novel is 'Acting Up', published by Hodder