The science of romantic comedy

The part they don't tell you in school about semen (from Scientific American):

Semen has a very complicated chemical profile, containing over
50 different compounds (including hormones, neurotransmitters,
endorphins and immunosupressants) each with a special function and
occurring in different concentrations within the seminal plasma. Perhaps
the most striking of these compounds is the bundle of mood-enhancing
chemicals in semen. There is good in this goo. Such anxiolytic chemicals
include, but are by no means limited to, cortisol (known to increase
affection), estrone (which elevates mood), prolactin (a natural
antidepressant), oxytocin (also elevates mood), thyrotropin-releasing
hormone (another antidepressant), melatonin (a sleep-inducing agent) and
even serotonin (perhaps the most well-known antidepressant neurotransmitter).

How it figures in plot (from an essay in The New York Review of Books):

Sex drive, for instance, is associated with the hormone testosterone in
both men and women. Romantic love is associated with elevated activity
of the neurotransmitter dopamine and probably also another one,
norepinephrine. And attachment is associated with the hormones oxytocin
and vasopressin. “It turns out,” [writer/scientist Helen Fisher] said, “that seminal fluid has
all of these chemicals in it. So I tell my students, ‘Don’t have sex if
you don’t want to fall in love.’”

But the easy part is the falling, isn't it?