Our bodies sometimes have different plans than our minds. Sexual activity triggers the brain to release that creates attachment between partners.
Scientists call this "the oxytocin paradox": you might want something casual, but your brain chemistry pulls you toward connection -
oxytocin—often called the "bonding hormone"Oxytocin works differently in men and women. Studies show single men under its influence become more drawn to short-term relationships with previously unfaithful women. Single women, however, lean toward long-term relationships with faithful men. This explains why casual encounters sometimes turn into something deeper than expected.
Oxytocin teams up with dopamine (the pleasure chemical) and can create unexpected bonds. A researcher points out that "It's not correct to think of oxytocin as the 'bonding hormone'... Oxytocin amplifies the face, smell, and voice of the person an individual is having sex with".