DID Romeo and Juliet, Cathy and Heathcliff, have more than love-sickness in common? It’s highly likely that they shared something else: an imbalance in their brain chemistry. Specifically, too much dopamine and too little serotonin.

Scientists can now show that infatuation, those early days, weeks, months (rarely years) of falling and being in love, is more or less the same as a form of mental illness. MRI technology can now track the chemical changes in the brain that ensue when love is "activated" – and they are similar to those which occur in people suffering from Obsessive Compulsive Disorder (OCD), a pattern of thinking and behaviour characterised by obsession, anxiety, compulsion and temporary relief.

The same technology can reveal the chemical path of true love as it evolves through different stages such as lust, romance and the forming of deep attachment. The early, heady days of falling in love are marked by a huge surge in the production of Dopamine: the chemical that creates feelings of euphoria and hugely increases our energy levels and sharpens our focus so efficiently that we can lock on to our object of desire like Exocet missiles. That’s why when we first fall head over heels, we don’t really feel the pain of anything. A toothache? It can wait.

Sprained an ankle when engaged in the acrobatics of love? No problem, put some frozen peas on it later. Haven’t slept in 48 hours? Who cares. You are numb to everything, save the object of your desire. Dopamine is a fantastic pain-blocker and it helps us to take risks that we wouldn’t ordinarily dream of taking. It can lead to recklessness and folly, just as much as it can beat the fastest path to exhilaration and orgasmic joy.

But just as a cocaine or alcohol addict becomes increasingly immune to their drug and needs more and more of it to attain the same high, we also become immune to the dopamine that creates the intense highs of infatuation and passionate love. Inevitably, this means those magical, romantic days are numbered.

And necessarily so. How would the world look if we were in a constant state of euphoric love? My bet is that there would be a lot fewer houses, bridges, trains and planes built. We would all be building imaginary castles in the air and there wouldn’t be much concrete stuff going on. The obsessive nature of infatuation means we have little time for much else, save the object of our heart’s desire.

It’s the same all over the globe. Falling in love is not a Western thing. Its expression varies across different cultures, but the experience of falling in love is an integral part of being human and regardless of race or social status, it produces the same effects in all of us. Young love can be more blind and mercurial than older love, but falling in love at any age packs a very powerful punch to our grasp on daily routine and transforms, albeit temporarily, the way we experience the world around us.

But it doesn’t last. If love does endure, the way we express it and experience it changes significantly. It becomes more grounded, there are fewer and less intense highs and lows, it becomes, ultimately, part of our domestic structure. We propagate the species by having and raising children. And as these changes happen, the brain chemistry of our love also changes. As we settle into a comfortable bond with our partner and feel more secure in the relationship, we produce more Oxytocin, the chemical that promotes bonding between mother and baby. A mother who is lacking in Oxytocin is more likely to neglect her child.

The more secure we feel with our loved one, the more serotonin we produce. Serotonin helps us to feel calmer. It stabilises our mood and increases feelings of wellbeing. The downside, as anyone on anti-depressants will tell you, is that serotonin significantly diminishes sex drive, making us much more likely to cosy up for friendly cuddles on the sofa, rather than revel in the abandon of adventurous, uninhibited sex.

Falling in love is easy. We don’t have to do much except surrender to it and let ourselves be taken along by it. Keeping love alive is much more challenging. Some people manage it. Some don’t.

As the bio-chemistry of love waxes and wanes, we find ourselves struggling to accept the loss of romance, the fading of sexual passion, the tyranny of domesticity. The trick seems to be in learning to adapt to the changes without running for the hills. In adapting, there is always the possibility that love can thrive, still be capable of surprises, throwing up the odd night of passion even if the chemistry isn’t quite what it used to be.

The hills are full of those who refuse to tolerate a drop in their dopamine levels. You hear them howling and stamping their feet as they get ready to come down to lock on to their next object of desire.