A New York University did a study about how happy girls are after they have perform oral sex on a guy and swallow the evidence of his passion.

Seems they get a little Cock-tail (sorry for the pun) of happy boosting chemicals found in semen which include:
Cortisol – known to increase affection
Estrone and oxytone – which both elevate your mood
A hormone which releases thyrotropin, an anti-depressant
Melatonin – a sleep inducing agent
Serotonin – an anti-depressant...