The body builds up tolerance to these chemicals and it then takes more of the substance to get that special feeling of infatuation. In the case of enduring romances, the continued presence of a partner stimulates production of endorphins which are soothing substances and natural pain-killers. Too much stress can lead to permanently low levels of serotonin and high levels of norepinephrine . Low levels of serotonin can create aggression. Serotonin usually acts as an inhibitor.
Low levels make it hard to get a grip.
Emotions play an important role when learning and memory circuits are being established. The strength of the synaptic connection that is formed when a memory is created is directly connected to the strength of the emotions at the time. This is why trivial events are easily forgotten and why emotional patterns are hard to change. During adulthood the neural connections continue to change more slowly and less frequently. Instead of new pathways being formed, the existing pathways are reshaped.