This little puppy has been linked to bonding and attachment behaviours in both animals and humans.
Oxytocin is a peptide of 9 amino acids that has both peripheral (hormonal) actions through secretion from the pituitary gland, and actions within the brain reflecting it's release from centrally projecting Oxytocin neurons in the amygdala, ventromedial hypothalamus, septum and brainstem.
It's actions include the 'let-down' reflex in lactating mammals (whereby breast milk is let down into collecting chambers in the mammary glands ready for suckling), cervical dilation and Uterine contraction during labor, and other bonding and maternal behaviours in mammals. More recent research has investigated the connections between Oxytocin and sexual response in humans, and while the jury is still out it seems that this little bundle of amino acids has a key role to play in sexual arousal and orgasm (when injected into the cerebrospinal fluid of male rats it causes spontaneous erections). Furthermore, some studies have hinted that it also plays a part in increasing empathy and trust and reducing fear.
So.... next time you hear you heart go 'Boom-Boom-Chick-a-Wow-Waa' in the presence of some guy or girl... don't fret, just blame your Oxytocin.
Edit: This is a rather fun link too... The Cuddle Hormone ... I particularly like the quote;
"You first meet him and he’s passable," Witt said of the phenomena. "The second time you go out with him, he’s OK. 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."