Prolactin is also involved. It, testosterone and estrogen are homones, oxytocin and vassopresin are peptides, dopamine, seratonin, [and several others] are neurotrasmitters. For men testosterone rules libido and aggression. Women, they still don't really know. Some repond to testosterone increases, others don't and you have to be careful or you pick up male characteristics if you over do it. Lowering prolactin and stimulating dopamine seems to intensify, but not necessarily create libido. Oxytocin seems to help bonding in both sexes, but not necessarily libido. NO seems to get the equipment working. Bing fertile seems to increase libido for women. Being on thepill seems to decrease it as do ssri's, but some have pradoxical increases in libido. when first falling in love you get a big spike of dopamine which is an excitatory neuro transmitter (along with a bunch of other stuff). THat is what keeps you awake, makes you grind your teeth and makes you an ocd poster child. That wears off eventually and seratonin/oxytocin takes over as the main juice so you just feel good hanging out together.