Have you ever wondered why people behave so irrationally in love?    Or even how love leads to violent attachment for some couples? One reason has to do with the brain substrates of romance (namely, dopamine and limbic pathways) being strong motivators for action. That is, we find sexual cues pleasurable in the same way that food and drugs are rewarding, and this drive for sexual release can be quite powerful- able to elicit craving that overrides reason (e.g. we temporarily lose prefrontal control over behavior, in favor of limbic emotion). Biological anthropologist Helen Fisher describes the role of dopamine neurotransmitter in romantic attachments here:

A striking phenomenon in our society is pair -bonding and how people choose mates for life. It turns out that other animals can also exhibit monogamous relationships, and scientists have analyzed the brain chemistry of such mammals. Voles are small furry rodents with a fidelity gene (see BBC News here) and have provided some intriguing data (link here) because they have differences in hormone levels of oxytocin and vasopressin (data here) that underlie their monogomous sexual behavior. It turn out that humans have these exact same hormones, which are elevated during romantic encounters (see “The Trust Molecule” in WSJ). 

So what about those unhealthy relationships of poorly-matched human couples, as manifested by high divorce and cheating rates (see WSJ‘sShow Me the Love… or Not). Psychiatrist Amir Levine hypothesizes why some people are addicted to abusive romances. It turns out that our adult relationships often mirror our earlier experience with maternal bonding (either attachment or avoidance). See this phenomenon discussed in NYT‘s “The Brain on Love

More information about the brain chemistry of social attachment and trust (even outside of a romantic context):