Syracuse University professor Stephanie Ortigue compiled MRIs from people around the world who indicated they were either in love or were "experiencing maternal or unconditional love." Using that data, she was then able reveal a "passion network" in our brains, a roadmap of spots that "release neurotransmitters and other chemicals in the brain and blood that create the sensations of attraction, arousal, pleasure … and obsession."
Above is a depiction of the various points on the brain that compose this newfound network, the specifics of which can be found here. Among other interesting machinations of a body in love, cortisol levels in the blood increase, jacking up stress and alertness but lowering pain sensitivity. Also, the brain's serotonin levels decrease, subsequently spiking obsessive thinking and aggression.
Update: Image removed at the request of the owner.