This explains a lot

You've got to appreciate a news story that begins "New love can look for all the world like mental illness, a blend of mania, dementia, and obsession . . . that could almost be mistaken for psychosis." Other good lines:

. . .researchers in New York and New Jersey argue that romantic love is a biological urge distinct from sexual arousal.

It is closer in its neural profile to drives like hunger, thirst or drug craving, the researchers assert, than to emotional states like excitement or affection.

And this

Dr. Helen Fisher, an anthropologist at Rutgers University and the co-author of the analysis. "And when rejected, some people contemplate stalking, homicide, suicide. This drive for romantic love can be stronger than the will to live."

Yeesh. I will refrain from interjecting the thousand personal anecdotes that seem frighteningly relevant here, except to say there is some limited relief in understanding that my ex-es were merely acting under biological imperative.