A U.S. News and World Report article explains the importance of the vagus, a nerve that threads through your whole body. Everyone knows that jumpy, sort of sick feeling in your stomach. When too many happy chemicals like PEA and dopamine flood your brain, they head straight for the limbic system. When The Honeymoon's Over Some scientists believe that after a certain period, from 18 months to 4 years, one's body gets used to these love stimulants. In this phase of the relationship, your brain produces endorphins, brain opiates more like morphine than speed.
Relationships take work.
They just do. In the end, even hard-core scientists agree that chemistry isn't everything. Culture, circumstances, personality, and scores of other variables help decide who turns your head and who leaves you cold. Experiment at MichaelAngel's Hopelessly Romantic Personals .