From the Daily Mail:
"Other research backs up this idea that women make less serotonin than men. Scientists at the University of Montreal found that men’s brains, on average, make 52 per cent more than women. The reason may be tied to differences in male and female sex hormones.Perhaps this explains why Semen Makes Women Happy.
While the findings about women producing less serotonin are controversial, ‘oestrogen and testosterone, the main sex steroid hormones, have long been known to affect behaviour’, according to the British Society for Neuroendocrinology.
However, oestrogen can have a bigger effect on women due to their ever-fluctuating hormone levels, says Dr Andrei Novac, clinical professor of psychiatry at the University of California Irvine School of Medicine."