It certainly would have been adaptive for ancestral man to have a chubby wife during stressful times of famine. Not only would she have had more calories to burn, and thus more energy and endurance, but since fat stores estrogen, she would have remained fertile for longer.

Real competition can drive up testosterone, which boosts libido.

A lot of people have been romantically in love with somebody who they feel wasn't appropriate to marry.

We evolved in a tropical climate where the smells of plants and flowers were all around us. We spent a lot of time in the trees with a lot of sunlight and no clothes.

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.

There were real reasons that you were attracted to somebody originally. The brain doesn't pick willy-nilly. Unless you part ways hating each other for some reason, that mechanism could get triggered again. You can literally fall in love again.

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.