There is a school of thought that indicates the brain chemical, Dopamine rushes through the brain allowing one to feel great while norepinephrine stimulates the production of adrenaline (pounding heart), and phenylethylamine brings forth euphoria leaving the individual to perhaps throw caution to wind. 

If you find yourself being stalked or perhaps being the perpetrator, consider that there may have been shift in the brain's equilibrium resulting in the limbic system separating itself from the cortex causing infatuation.

 The body builds up tolerance to these chemicals therefore requiring more of the "substance" to get that special feeling of infatuation. People who jump from relationship to relationship crave the intoxication of falling in love and may be "attraction junkies". In the case of enduring romances, the continued presence of a partner stimulates production of endorphins which are soothing substances and natural pain-killers.