From the Economist (of all places)--
Scientists are finding that, after all, love really is down to a chemical addiction between people:
The prairie vole is a sociable creature, one of the only 3% of mammal species that appear to form monogamous relationships. Mating between prairie voles is a tremendous 24-hour effort. After this, they bond for life. They prefer to spend time with each other, groom each other for hours on end and nest together. They avoid meeting other potential mates. The male becomes an aggressive guard of the female. And when their pups are born, they become affectionate and attentive parents. However, another vole, a close relative called the montane vole, has no interest in partnership beyond one-night-stand sex. What is intriguing is that these vast differences in behaviour are the result of a mere handful of genes. The two vole species are more than 99% alike, genetically.
But it's not all in the genes, and are humans hard-wired like prairie voles or montane voles?:
Of course, love is about more than just genes. Cultural and social factors, and learning, play big roles. Who and how a person has loved in the past are important determinants of his (or her) capacity to fall in love at any given moment in the future. This is because animals—people included—learn from their sexual and social experiences. Arousal comes naturally. But long-term success in mating requires a change from being naive about this state to knowing the precise factors that lead from arousal to the rewards of sex, love and attachment. For some humans, this may involve flowers, chocolate and sweet words. But these things are learnt.
Read the article for much more than I can explain about hormones, smells, brain activity, etc.
