When an individual experiences all three stages, a very strong bond is created. She concluded that "love is not an emotion but a physiological drive as powerful as hunger" and that "romantic love is actually a basic drive that has evolved for the purposes of mating and reproduction".

Lust/erotic passion

Oestrogen and testosterone become active in our bodies for the first time, just after puberty, and create the desire to experience 'love'.

These desires, also known as lust, play a pivotal role in our emotions both during puberty and throughout our lives.

Testosterone is a libido-enhancing, energy-producing chemical secreted in the testes of males and ovaries of females. On average, men produce about 20 times more testosterone than women.

According to an article by Lisa Diamond, titled 'Love and sexual desire' (Current Directions in Psychological Science, Vol 13 No 3), lust and romantic love are two different things caused by different underlying substrates. Lust evolved for the purpose of sexual mating, while romantic love evolved because of the need for infant/child bonding. She concluded that "even though we often experience lust for our romantic partner, sometimes we do not".

Sexologist John Money also contrasts love and lust in this way: "Love exists above the belt, lust below. Love is lyrical. Lust is lewd."

Pheromones, looks and our own learned predispositions for what we look for in a mate play an important role in whom we lust after as well. Without lust, we might never find that special someone. But, while lust keeps us "looking around", it is our desire for romance that leads us to attraction.

Romantic love/attraction

While the initial feelings may (or may not) come from lust, what happens next - if the relationship is to progress - is attraction.

When attraction, or romantic passion, comes into play, we often lose our ability to think rationally. The old saying 'love is blind' is really accurate in this stage. We are often oblivious to any flaws our partner might have. We idealise them and cannot get them off our minds.

When people fall in love, they cannot think of anything else. They might lose their appetite, need less sleep and spend hours daydreaming about their new lover.

That euphoric surge that comes when we fall in love is due to the dopamine, norepinephrine and phenylethylamine that are released in our brain.

Dopamine is thought to be a 'pleasure chemical', which produces a feeling of bliss.

Norepinephrine is a euphoric chemical, which stimulates the production of adrenaline that elevates your blood pressure, causes your heart to flutter and produces elation, intense energy, sleeplessness, craving, loss of appetite and focused attention.

Phenylethylamine is a naturally occurring amine similar to amphetamine that fuels desire.

Dr Fisher posits: "The human body releases this cocktail of love only when certain conditions are met and ... men more readily produce it than women because of their more visual nature."

The feelings of passionate love, however, do lose their strength over time. Studies have shown that passionate love fades quickly and is nearly gone after two or three years. The chemicals responsible for "that loving' feeling" (adrenaline, dopamine, norepinephrine, phenylethylamine, etc) dwindle.

Suddenly your lover has faults. Why has he or she changed? Actually, your partner probably hasn't changed at all; it's just that you're now able to see him or her rationally, rather than through the blinding hormones of infatuation and passionate love. At this stage, the relationship is either strong enough to endure, or the relationship ends. In this stage, couples spend many hours getting to know each other. If this attraction remains strong and is felt by both of them, they usually enter the third stage: attachment.

Attachment/companionate

The attachment or commitment stage is long-term love. You've passed fantasy love and are entering into real love. This stage of love has to be strong enough to withstand many problems and distractions. Attachment is the bond that keeps couples together when they have children and enable them to cope with the other challenges of life.

Two hormones released by the nervous system in this stage are thought to play a role in social attachment, namely oxytocin and vasopressin.

Oxytocin, sometimes known as 'the cuddle chemical', helps bond the relationship. According to researchers at the University of California, San Francisco, this hormone has been shown to be "associated with the ability to maintain healthy interpersonal relationships and healthy psychological boundaries with other people". When it is released during orgasm, it creates an emotional bond - the more sex, the greater the bond. Oxytocin is also associated with mother/infant bonding, uterine contractions during labour in childbirth and the 'letdown' reflex necessary for breastfeeding.

According to birthpsychology.com, scientists now think that both genders release this nurturing hormone when touching and cuddling,

Vasopressin: Another important chemical in the long-term commitment stage is an important controller of the kidney and its role in long-term relationships was discovered when scientists looked at the prairie vole.