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Studies show that people at an early stage of love have lower levels of serotonin, which is associated with feelings of happiness and well-being, and higher levels of cortisol, associated with stress. This is strikingly similar to those people who have Obsessive-Compulsive Disorder, which explains why we act so out of character when we fall for someone. It works the other way around as well – people with lower levels of serotonin fall in love and get into sexual relationships quicker than others. Check out this vivacious video that explains the theory.