Unpredictable love.

There's a nice pieces in the "Gray Matter" series in the New York Times by Richard Friedman that argues that it is our evolved motivational machinery (that drives us to seek unpredictable rewards - i.e., transient reinforcement) more strenuously than predictable ones, that underlies the fickleness of love.

Shakespeare warned women that “men were deceivers ever; one foot in sea and one on shore, to one thing constant never.”

...how commonly people complain that they love someone who always disappoints them...This kind of amorous attachment is like gambling — except that the currency is affection and sex. The key is that the reward is unanticipated, which makes it particularly powerful and alluring to our brains.

Many experiments have shown, in both animals and humans, that intermittent rewards cause more activation and dopamine release in the brain's reward circuits than predictable rewards.

The brain’s reward circuit has evolved over millions of years to enable us to recognize and extract various rewards from our environment that are critical to our survival, like food and a suitable sexual mate. Unlike predictable stimuli, unanticipated stimuli can tell us things about the world that we don’t yet know. And because they serve as a signal that a big reward might be close by, it is advantageous that novel stimuli command our attention.

The article centers on the work of psychiatrist Gregory Berns:

One of the curious things that Professor Berns found was that most of his subjects couldn’t tell the difference between the predictable or unpredictable condition in which the reward was given...Since unpredictable rewards cause more dopamine release than predictable ones and more dopamine means more pleasure, one implication of this study is that people experience more pleasure with unpredictable rewards than with predictable ones — but they may not be consciously aware of this fact...Not just that, but there was essentially no relationship between the subjects’ stated preferences and the observed activity in their reward circuit. This suggests that our reward pathways may not only be activated without our recognition, but perhaps even in ways that are contrary to what we think we prefer...These data might explain, in part, the paradox of people who complain constantly about their unreliable lovers, but keep coming back to them, time and again.