Scientists at Bonn University Hospital examined the function of oxytocin on 40 men in relationships, dosing them up with the hormone via nasal sprays.
When the men who were under the influence of the hormone saw pictures of their partners, their brains' reward centres were activated - almost as if they had taken drugs.
"This biological mechanism is very similar to a drug," said Professor René Hurlemann who led the study. His results were published in the Proceedings of the National Academy of Sciences journal in the United States.
The men's reward centres were not activated when they were shown pictures of other women - which would lead the men to seek out their partners rather than other women, in order to experience the effect.
"In love, just as in the consumption of drugs, people are seeking the stimulation of the reward system in the brain," said Hurlemann.
He said this could also explain the tendency of partners to touch each other a lot. "Why do couples touch each other so often? Because in this way the oxytocin level is kept high, and the connection between them further stabilized."