|Artistic rendition of oxytocin, C43H66N12O12S2, a 5-element neurochemical, the so-called 'cuddle chemical', with heart shaped letter Os.|
The effect of oxytocin to mediate spatial tendency, to the effect that people stand closer to those they like than to those they abhor, was discovered in 1968. In the early 1980s, oxytocin release was being associated with breastfeeding.
The “oxytocin theory of love” with specific focus on marriage formation and marriage dissolution seems to have gained fame with the publication of the 1992 studies of American behavior neuroscientist Thomas Insel who studied the differences in mating behaviors and corresponding oxytocin levels in two types of voles: prairie voles (prairie living voles), wherein adults tend to be monogamous, wherein both parents nurture their young, and mountain voles (mountain living voles), wherein adults tend to be more promiscuous and parent engage in less caretaking of the young. Insel found that oxytocin levels tend to be high in the monogamous prairie voles and low in the promiscuous mountain voles.  Oxytocin seems to have began to be called the ‘cuddle chemical’ or ‘love hormone’ into the 1990s the 2000s.
The neurochemical oxytocin is a bonding molecule (hormone), with a number of characteristic traits: high levels correlate with strong pair-bonding; levels rise during touching, kissing, and foreplay, and peak during orgasm. Associated with male-female attachment. Is made in the hypothalamus, ovaries, and testes. Is released in women during the birthing process. Initiates contractions of the uterus, stimulates the mammary glands to produce milk, and stimulated bonding between a mother and her infant. At orgasm, levels increase dramatically in women. Released during stimulation of the genitals and nipples. Is secreted in response to the crying of the infant.
Thinking about a loved one will cause levels to rise. It reduces stress. It makes people forgetful and diminishes the capacity to think and reason. Is secreted in the posterior lobe of the pituitary gland. When given to females around ovulation, it heightens their efforts to contact males and intensifies lordosis, i.e. sexual presenting. When given to males, they will develop maternal behaviors and infant guard ferociously; when levels are artificially blocked in males, they will neglect their infants and may even eat them. Improves erection, speeds ejaculation, enhances contractions of penile tissue, and increases ejaculation volume and sperm count; conversely, lowered levels reduces sperm count. Penis-vaginal contact and penetration increases levels in both sexes. 
In 2004, American neuroeconomist Paul Zak's lab discovered that oxytocin allows us to determine who to trust. Zac’s obsession with finding the chemical basis for morality has led him to write the 2012 book The Moral Molecule. 
1. Thims, Libb. (2007). Human Chemistry (Volume Two) (ch. 12: Bond History and Neurochemistry, pgs. 469-513, esp. pgs 503-04). Morrisville, NC: LuLu.
2. (a) Lewis, Thomas, Amini, Fari, and Lannon, Richard. (2001). A General Theory of Love (pg. 96). Random House.
(b) Thomas Insel – Wikipedia.
3. Zak, Paul J. (2012). The Moral Molecule: the New Science of What Makes us Good or Evil. Dutton Adult.
● Kuchinskas, Susan. (2009). The Chemistry of Connection: How Oxytocin Response Can Help You Find Trust, Intimacy, and Love. New Harbinger Publications.
● Breuning, Loretta G. (2012). Meet Your Happy Chemicals: Dopamine, Endorphin, Oxytocin, and Serotonin. CreatSpace.
● Oxytocin – Wikipedia.