Maybe it's not similar interests, horoscope signs, looks, or proximity that make women and men fall in love. According to evolutionary scientists, when people throw up their hands and say "it was just chemistry," they may be on to a fundamental factor in mate choice.
Subtle chemical signals, or pheromones, have long been known to draw pairs together within the same species, and for a specific reason. In mice, for example, experiments showed that pheromones acted as attractants between males and females who were genetically similar except that they differed in a certain type of immune system gene. That difference is actually a survival benefit: The combination of two individuals' different MHC (major histocompatibility locus) genes gives their offspring an advantage in beating back disease organisms.
So the mice could smell a genetic difference. But could modern humans, who aren't known for a particularly good sense of smell, also make that distinction?