From Scientific American:
“Neuroscientist Gert Holstege of the University of Groningen in the Netherlands and his colleagues attempted to solve the male side of the equation by asking the female partners of 11 men to stimulate their partner’s penis until he ejaculated while they scanned his brain using positron-emission tomography (PET). During ejaculation, the researchers saw extraordinary activation of the ventral tegmental area (VTA), a major hub of the brain’s reward circuitry; the intensity of this response is comparable to that induced by heroin.”
Alternatively, heroin is about as awesome as sex!