Whereas love tends to be based on longer lasting emotions of trust, security, and compassion and is what keeps two people, and in some cases animals, together for extended periods of time. Love gives time for mating and afterwards the raising of offspring. Love and lust are biochemically separate systems, a two-headed system that has been hard wired into our brains through millions of years of evolution. It is often when the two systems are not in synch that couples experiences marital strife. (Sinha, Gunjan http://www.popsci.com/).
In a recent study using prairie voles, Lowell Getz, a former professor of ecology, ethnology, and evolution at the University of Illinois, was able to isolate the cause of long-term relations between male and female voles, a hormone called oxytocin. It is a chemical that is existent in mammalian brains and can sometimes promote bonding between males and females of the same species and females and their offspring. Though he was not the first to discover the hormone itself Getz was able to prove, with the help of neuroendocrinologist and colleague Sue Carter, that an increased level of oxytocin in females caused them to be less choosey about their mates and more affectionate towards them (more cuddling and licking occurred, they also avoided strangers more often) than untreated females. Conversely, decreasing the oxytocin present in the female voles with an oxytocin-suppressant had just the opposite affect; the females were now less likely to stay with a mate and deserted those they were presently attached to. This chemical is present in humans, both male and female, and studies have shown that the levels of oxytocin increase during sexual arousal and jump to a large degree during orgasm. The higher the levels of oxytocin present in the bloodstream during intercourse the more intense the orgasm becomes. (Sinha, Gunjan http://www.popsci.com/).
After mating occurs between two voles, another biochemical kicks in, vasopressin.