Hormones

Oxytocin:
http://newsinhealth.nih.gov/2007/February/docs/01features_01.htm
Oxytocin makes us feel good when we’re close to family and other loved ones, including pets. It does this by acting through what scientists call the dopamine reward system.


Androstadienone:
http://www.sfgate.com/cgi-bin/article.cgi?f=/n/a/2007/02/10/state/n140520S56.DTL
A chemical in male sweat can boost mood, brain activity and sexual arousal in heterosexual women, according to a new study released just in time for Valentine's Day.

And a fun thing on the substantia nigra:
http://en.wikipedia.org/wiki/Substantia_nigra