When a male vole repeatedly mates with a female, a hormone called vasopressin is released in his brain. The vasopressin binds to receptors in a part of the brain called the nucleus accumbens, and the binding mediates a pleasurable feeling that becomes associated with that female. This locks in the monogamy, which is known as pair-bonding. If you block this hormone, the pair-bonding goes away.
David EaglemanIn our current understanding of science, we can't find the physical gap in which to slip free will - the uncaused causer - because there seems to be no part of the machinery that does not follow in a causal relationship from the other parts.
David Eagleman