Seeing a lover prompted activity in four brain regions that were not active when looking at pictures of a friend, and caused a significant reduction in the activity of one other area... Two active areas lay deep in the cortex: the medial insula, which may be responsible for "gut" feelings, and a part of the anterior cingulate, which is known to respond to euphoria-inducing drugs... The inactive region was in the right prefrontal cortex--the region that is overactive in depressed patients.
It's probably only a matter of time before this appears on the market, as a device which tells you whether your partner is really in love with you, or just using you for money or sex or somesuch.