"adrenaline, dopamine and serotonin." He blurred out. Love is a chemical reaction, Sherlock figured. However, he could tell by John's puzzled face that he didn't understand what Sherlock was on about.
"Adrenaline, you start sweating when you see the person you like. The heart starts racing, pumping more of it through your veins. The mouth goes dry, leaving the tongue thick and dry, making it more complicated to speak." He explained. Sherlock had felt all of this when he ran in here and saw John. He had felt it when they first saw each other again.
"Dopamine has the same effect on the brain as cocaine." He continues, leaving John no time to interrupt and his words came out like a water fall, his voice building up from low and deep, to rather fierce by the end. "This chemical stimulates ‘desire and reward’ by triggering an intense rush of pleasure. Increased energy, less need for sleep or food, focused attention and exquisite delight in smallest details of the relationship."
"And last Serotonin. One of love's most important and dangerous chemicals causing you can only think about one certain person. Serotonin levels of new lovers are equivalent to the low serotonin levels of Obsessive-Compulsive Disorder patients. John can't you see what this is doing to me?" He snapped at the end.