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Sexual activity causes the release of the serotonin, commonly called the "happiness hormone" because it regulates our mood. Healthy levels provide us with a general sense of well-being, and a lack of serotonin is often associated with depression. However, anti-depression drugs attempt to increase the amount of this hormone in a person's system, but they can also reduce sex drive. If your serotonin levels are normal, it's best to get your happiness hormone naturally, which can be done through sex, exercise, or eating carbs.