There are a lot of chemicals racing around your brain and body when you're in love. That initial giddiness that comes when we're first falling in love includes a racing heart, flushed skin and sweaty palms. Researchers say this is due to the dopamine, norepinephrine and phenylethylamine we're releasing. Dopamine is thought to be the "pleasure chemical," producing a feeling of bliss. Norepinephrine is similar to adrenaline and produces the racing heart and excitement. These two chemicals produce elation, intense energy, sleeplessness, craving, loss of appetite and focused attention. The brain scans showed increased blood flow in areas of the brain with high concentrations of receptors for dopamine associated with states of euphoria, craving and addiction. High levels of dopamine are also associated with norepinephrine, which heightens attention, short-term memory, hyperactivity, sleeplessness and goal-oriented behavior. In other words, couples in this stage of love focus intently on the relationship and often on little else. People in love have lower levels of serotonin and also that neural circuits associated with the way we assess others are suppressed. Vasopressin, an antidiuretic hormone, is another chemical that has been associated with the formation of long-term, monogamous relationships. Endorphins, the body's natural painkillers, also play a key role in long-term relationships. They produce a general sense of well-being, including feeling soothed, peaceful and secure. The feelings of passionate love, however, do lose their strength over time. Studies have shown that passionate love fades quickly and is nearly gone after two or three years. The chemicals responsible for "that lovin' feeling" (adrenaline, dopamine, norepinephrine, phenylethylamine, etc.) dwindle. Suddenly your lover has faults. Why has he or she changed, you may wonder. Actually, your partner probably hasn't changed at all; it's just that you're now able to see him or her rationally, rather than through the blinding hormones of infatuation and passionate love. At this stage, the relationship is either strong enough to endure, or the relationship ends. References: http://people.howstuffworks.com/love6.htm