"The "cuddle chemical" oxytocin may help partners stay faithful by making monogamy more rewarding. Researchers at Bonn University Medical Center in Germany found that if they administered oxytocin to men, their brain's reward centers became highly activated when they looked at pictures of their partners. In work published in PNAS, the researchers administered oxytocin in a nasal spray to 40 heterosexual men in a committed relationship. Compared to controls, the men who received the oxytocin showed higher levels of activity in the ventral tegmental area and the nucleus accumbens, which serve as the reward centers of the brain, when looking at pictures of their partners, and rated them as more attractive. This effect was specific for a romantic partner and did not occur when the men looked at pictures of colleagues and acquaintances. The researchers believe this surge of reward is one of the reasons that couples are able to stay monogamous for long periods of time."


http://www3.uni-bonn.de/Press-releases/oxytocin-leads-to-monogamy