Once testosterone gets the job done of attracting two people to each other and igniting their sexual energy, pleasure-inducing dopamine is released during sexual intercourse [source: Discovery Channel]. By powering the brain's limbic reward system, this chemical cultivates the addictive satisfaction that comes with euphoria derived from eating a delectable meal or making love or even using cocaine. That neurological treat dopamine doles out in the early stages of romantic attraction is partially responsible for the thrill we get when seeing the object of our affection and the craving to be with him or her in between. Indeed, studies have demonstrated that the brain, thanks to dopamine and its neurochemical cohorts, processes and manifests love much like an addiction, compelling people to settle down and reproduce [source: Fisher et al].
Though high levels of dopamine are characteristic of early stage romance and obsession, its presence is also a hallmark of satisfying, lasting relationships years later. Functional MRI scans of couples married at least 21 years who reported being madly in love showed above average activity in the brain's dopamine factory, the ventral tegmental area [source: Lite]. So while oxytocin and vasopressin are important for establishing monogamy, dopamine ensures that Pablo Neruda's "endless simplicity of tenderness" remains long after the honeymoon phase fades.