Diane Witt, a researcher at Binghamton University, proposes that the release of oxytocin can be classically conditioned to the sight of certain people. Recall the Nobel Prize-winning Russian scientist Pavlov and his dogs. Dogs salivate when they are exposed to food--it plays an important role in the digestive process. Pavlov began ringing the bell every time he fed his dogs, and after a while the sound of the bell alone caused the dogs to salivate. The dogs had been classically conditioned to salivate at the sound of a bell. Witt believes that, in a similar way, oxytocin can be classically conditioned to be released by the brain with exposure to certain partners.
For example, a woman meets someone and on the first date she decides he doesn't match up to her ideal--Clint Eastwood--but he's still acceptable enough to date a few more times. Eventually she decides to have sex with him--and oxytocin is released, so she experiences that "oohhh so good" feeling. After having repeated sex, and oxytocin releases, with the same man, she forms a conditioned association. Pretty soon, just seeing the guy can cause her brain to release oxytocin--without even having sex. Suddenly "Mr. Acceptable Enough" becomes "Mr. Can't Live Without." Some researchers believe that prolonged attachment with a given person actually causes chronically high levels of oxytocin and its close hormonal relative vasopressin, which could feasibly help maintain long-term relationship bonds between women and men.
Problem is, you have to be acceptable in the first place.