There's all kinds of reasons that you fall in love with one person rather than another: Timing is important. Proximity is important. Mystery is important. You fall in love with somebody who's somewhat mysterious, in part because mystery elevates dopamine in the brain, probably pushes you over that threshold to fall in love.

Romantic love allows you to focus mating energy. Attachment sustains that relationship as long as necessary to raise your baby.

The main characteristics of romantic love are craving: an intense craving to be with a particular person, not just sexually, but emotionally.

For so many generations, a woman's only career path was to marry well and to marry up. Those days have changed.

The Great Depression of the 1930s saw more American unmarried women working from nine to five, mostly in repetitive, boring, subordinate, dead-end jobs. But the number of working women doubled between 1870 and 1940. During World War II it doubled once again.

We spend our lives trying to get along with people so we can keep our jobs, keep our marriages together, so that we can raise our kids properly.

We're apt to fall in love with those who are mysterious and challenging to us.