In addition, studies show that oxytocin in females, as well as the closely related vasopressin in males, is key to pair bonding. "You first meet him and he's passable," Witt said of the phenomena. "The second time you go out with him, he's okay. The third time you go out with him, you have sex. And from that point on you can't imagine what life would be like without him." "What's behind it?" she added. "It could be oxytocin." Since the release of oxytocin can be classically conditioned, after repeatedly having sex with the same partner, just seeing that partner could release more oxytocin, making you want to be with that person all the more, and you bond, she said. (oxytocin.org)