What is happening in the brains of lovers ?

The response of Jean-Didier Vincent, a member of the National Academy of Medicine. 

It is fashionable to oppose love, which is about feeling, and sex, which is summed up in the carnal act. This duality does not hold water. Love is always a matter of desire and it still has its headquarters in the brain. We feel before you act and the affect that comes from the body precedes the act dictated by the imperative force of desire. It is not in the heart, which is a performer that is fulfilled love (that is to say, sex), but in this organ which gather perceptions and decisions : the brain. It is a brain that lovers should burn in the bark of trees, not a heart. It's still in the brain are born enjoyment but also suffering, which are the acolytes of desire. Everything happens in the lower part of the brain, a funnel-shaped region called the hypothalamus and has roughly the size of a fingernail and gathers within itself the command centers of the major functions of the body reproduction, regulation of food intake and thirst, weight control, maintaining constant body temperature and finally sleep. It is increasingly penetrated by the nerve pathways of pleasure and pain, systems that give rise to desire and maintain. Briefly, in the "cellar" of the brain is all that is necessary for the maintenance of life. 
The hypothalamus is not only the narrow space crammed nerve centers of the famous functions, it is also a gland that discharges its products secretions into the bloodstream for irrigating the pituitary gland important SINCE it controls all the other hormones called by its stimulines. These in turn direct the secretion of sex hormones. This cascade commands reminiscent of the army, with the hypothalamus general, pituitary gonadal officers and soldiers. In return, these hormones act on their emitting cells for braking or for stimulating, what is called feedback (or feedback). At puberty, there is a chemical signal in the hypothalamus which triggers hormonal storm. We now know that this molecule is a peptide (a small protein), the kiss-peptide, resulting in the accelerated pace of luliberin which, in turn, causes massive release of sex hormones (progesterone and estradiol in woman, testosterone in men) making youth capable of reproduction. As for the kiss-peptide, a hormone secreted by the body fat cells, leptin triggers secretion. This explains why the age of puberty is related to fat mass. It is observed that girls just wrapped were settled earlier. Although complex watches that controls sex hormones!
  Desiring systems
  It is in the hypothalamus and adjacent brain regions that runs the music of sexual desire. Musicians (centers) are placed around and in front of the third ventricle, a space occupied by liquid contacting both lateral ventricles located in each hemisphere: an extensive network of ponds allowing the distribution of all kinds of substances , salts and hormones, in the heart of the brain. I remember that the hypothalamus is also the place that serves food, drink and sleep, all of which are not unrelated to sex. Centers of male behavior coexist with the centers of female behavior in the brains of both sexes. The dorsal-anterior area located in front of the hypothalamus plays a role in the male behavior. This center receives information from all sensory modalities and integrates the impressions which contribute to maintain the fire of desire and initiate precopulatory phase. It then ensures the continuation of the act itself. Dopamine is the major neurotransmitter involved in the activation center. Specialized in sex comes from dopamine neurons in the vicinity of the male center. It is different from that involved in general desiring systems that run through the base of the brain and irrigate the cerebral cortex. This specific dopaminergic system explains that sexual desire make relatively aloof from his companions pleasure. One reason for this independence could be its role in the evolution of species and the need to protect it with a special diet. This center does not act alone but in close relation with the regions that control motor and including sexual postures are also affected areas involved in memory - like humans, it is often remember - and emotions. In short, everything that gives meaning to love beyond the simple need for the species to reproduce. The center also has special connections with the smell whose role is essential in romantic encounter. The ventral and medial region of the hypothalamus is involved in female sexual behavior, but not exclusively so. It also participates in the control of feeding behavior and, more generally, the negative and painful aspects of animal behavior. The introduction of estradiol in this structure in a female castrated corrects loss of sexual behavior that followed the removal of the ovaries. But note that the implantation of estradiol in the nucleus in male castrated rats results in the latter adopting a female sexual posture and acceptance tributes same-sex partners! We must also reckon with the endorphins that oppose the pain caused by the female sex. The essential mission of the sexual centers is to control the mechanics of sex triggered at the spinal cord by genital stimulation. The latter, under the direct influence of sex hormones (rut or heat), are sufficient to trigger the running of the sexual act with a female. These data refer of course animals, mainly rodents who served as an experimental model. These centers are at work in humans, but the genital areas are under strict control of the upper floors of the brain: Love is not a reflex. Sexual desire is in the head, not in the lower abdomen. It is in both male and female parts of the hypothalamus that is the integration of sensations, gestures of love, emotions, anxiety too, that will trigger or inhibit the sexual act itself. As seen, the masculine and feminine exist in the brain and throughout the life of the human. You can imagine hypothetically that the two structures are the scales of a balance beam which indicate the sexual orientation of the individual, in other words the appeal of a male or female partner to be straight or gay, nothing seems preordained. Now consider what happens in the brain and body during mating of the two lovers. A hormone secreted by large neurons of the hypothalamus, oxytocin is released in the brain in response to stimulation of the genital area. It accompanies the rise of the couple desire / pleasure in causing enhancement of the dopaminergic systems. Discharged into the bloodstream by the pituitary gland, it accentuates the rhythmic contractions of the genital muscles which in turn amplify his release, reflex contributing to repackage the system to orgasm. It keeps its state in the brain, which manages the pleasure and orchestra organic manifestations. Words and molecules Potent analgesia blocks the painful nature of violent mechanical stimuli that combined sexes are needed, she silenced suffering for enjoyment can express themselves freely. The man and woman are, for a short time, confused in their being. Oxytocin does not limit its actions to its role in orgasm. Released into the blood during breastfeeding, it allows the ejection of milk in nipple sucking by the baby. Released in the brain, it facilitates the attachment between partners, which makes it unreasonably hormone called fidelity. It also plays a major role in maternal behavior. To conclude, the love for humans is hardly physically different from what can be observed in animals and it is enriched, however, all the mental and moral capacities of the species. The animal made love, but do not speak; human lives love and tells. The lover's discourse is part of the sexual act, as it is done in real or dreamed. The man does not just love, it speaks to the point that sometimes the sexual act disappears in favor of the love that the two lovers live novel. Level assigned to the language and imaginary parts, it is more so then chemical molecules as words, the most beautiful is the word "love." * Author among other "extraordinary Journey to the Center of the Brain", ed. Odile Jacob and "Gender explained to my daughter," ed. Threshold.