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Your brain and nervous system are networks of neurons signaling to each other. We talked about how neurons signal to each other across a synapse. Neurotransmitters are simple chemicals that cross a synapse to accomplish this signaling. They are released from one neuron’s axon, cross the synapse, and bind to receptors on the other neuron. Abundance or scarcity of certain neurotransmitters correlate to “feelings” you experience.
Serotonin is a neurotransmitter associated with feelings of calm and well-being. It is sometimes called 5-HT, which is a short form of its chemical name, and it is derived from the amino acid tryptophan. (This is the basis of the urban legend that turkey makes you happy via chemistry.)
Low levels of serotonin are thought to be associated with obsessive-compulsive disorder and depression. But what about the original question about love? Some studies have suggested that serotonin levels are low in people who are newly in love. This makes some sense, as this state is not calming, and does involve some compulsive behavior.
Incidentally, some anti-depressants called SSRIs work by blocking reuptake of serotonin back into the pre-synaptic neuron. This results in more serotonin in the synapse for a longer time, to achieve a calm feeling.