Everybody recognises the moment when the pulse races, the conversation just takes off and suddenly a stranger becomes the most fascinating person in the world. Later, we tell our friends that “we gelled”. What’s much harder to spot is when the gelling becomes setting and the boundless possibilities of falling in love become the everyday predictability of the same sex, same arguments and same social life.

Unfortunately, it is easy for a relationship to become something set in stone. “I used to worry about being rejected or how I’d perform in bed, but with Polly it all felt so natural,” says Martin, 28, an IT support worker. “However, five years later, I found that we were in a rut. I would snake my hand across her belly and she would either shift slightly in the bed or let out a soft sigh. The green signal. She would nibble my neck because I had told her that it turned me on. One morning, I realised that it did nothing for me and hadn’t for a while.” A few weeks later, Martin found the courage to talk to Polly: “We jazzed things up a bit and that helped for a while but we soon slipped back to the old routine.”

Carrie and James, a couple in their late twenties, gave up arguing because it never seemed to get them anywhere. “It would always start with an innocent comment: ‘We’re lost; perhaps we should ask someone’,” Carrie says. “He would snap back: ‘There’s nobody here to ask.’ I’d try to be rational but he’d lose it and I’d sulk, and we’d have five days of treading on eggshells.”

Although superficially the rows would be about different things, they always ended up in the same groove. “She’s always so critical,” says James. “She can’t give me credit for anything I do.” Carrie counters: “I’m not allowed to have opinions in my own home.” Rather than improving their relationship, giving up arguments has just left them stuck in their separate corners.

Marilyn, a 42-year-old mother of two, can see her marriage travelling down the same tracks for ever, and it terrifies her. “We do the same things, year in, year out. We go to America because we have family over there. At home, we see the same friends at the same dinner parties. I’ve told my husband how I feel and he gets worried. He’ll say: ‘Aren’t you happy? What’s wrong with Philadelphia?’ and there’s nothing wrong with it. I just long to break out.” Marilyn, and probably her husband, too, feel that their marriage has reached setting point and can see no other way of being different together.

So why do we get stuck and what can be done about it? On a superficial level, it is not hard to shake things up (see panel). The best way is to inject some fun back into the relationship. Unfortunately, while earning a living, running a house and raising children we can lose sight of our childlike sense of wonder and creativity.

However, as Martin and Polly found while trying to change their love lives, relationships have a tendency to settle back into the old pattern. This is the core of the problem. Strange as it seems, we are frightened of getting too close.

Our desire for intimacy might be strong but our fear of getting rejected is almost equally powerful. So we hold back and build up our defences as an insurance policy against pain. At the beginning of a relationship this “one foot in, one foot out” approach makes sense. We imagine that it will get easier to become intimate but often we become more scared. Our partners learn so much about our failings, as well as our strengths, from our life together that to share much more can feel like being swallowed up. Worse still, with somebody you know well, rejection feels more personal. The result is that we settle for the familiar. But unless a couple keeps pushing to enlarge their comfort zone, the relationship risks reaching setting point.

How can we deal with the fear? The first step is to accept that we are frightened. In counselling, I asked Martin how Polly would react if she knew about his sexual fantasies. “I don’t want to think about it,” he replied. But thinking about it is vital in turning fears into a concrete example. He started with the worst possible outcome and worked to the best. “She could laugh or think me a pervert,” said Martin, “but she could agree or even enjoy my fantasies.” With the fears in the open, we could look at where they might come from. Martin traced his fear of laughter back to locker-room teasing at school and attributed the idea that “nice boys don’t talk about sex” to his parents, who found the subject embarrassing. In fact, Martin had no reason to believe that Polly would react in this manner. “She has always been open and supportive,” he admitted. Unwittingly, he had begun the next final phase: giving credit to the other person.

Finally, he talked to Polly. Although she did not like all the ideas, some were a turn-on and she added a suggestion of her own. Meanwhile, Carrie and James found that they were not listening because they were afraid what the other might say. Ultimately, both couples discovered that they had nothing to fear but fear itself.

It is not necessary to start with a big subject, such as your sex life, to break free. Often a smaller area, such as your social life, will work but it should be something scary that pushes the boundaries of your comfort zone. A relationship is a living thing; it will never thrive in a cage.

Andrew G.Marshall is a psychologist specialising in counselling for couples

How to get unstuck

Couples become bored because they seek intimacy in a few set ways, so try something from a different category:

Tranquillity Go to the beach and skim stones across the waves together; find somewhere to play Pooh sticks.

Irresponsibility Generally, any forgotten childhood pleasures, such as pushing each other on the park swings or running down a hill singing Jack and Jill together.

Excitement Visiting a theme park and going on a white knuckle-ride together; a day at the races.

Sensual Going to a concert together; filling the house with fragrant flowers.

Achievement Go on a five-mile walk together or landscape the garden.