Fun fact: human men can lactate. Prolactin is the neurotransmitter responsible for breast milk production and is part of the pituitary gland. Oxycition-aka the chemical responsible for bonding-is released from suckling, causing the breast to release. It's not that men aren't able to produce the chemical-everyone does, especially during REM sleep-it's just that women who are in late pregnancy produce an insane amount, though changes in horomone levels due to recovery from things like starvation have been shown to cause lactation in men, and you can artificially induce it by lowering dopamine levels to produce breast milk. (See: Domperidone)