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.
Incognito: The Secret Lives of the Brain (ed. Vintage, 2011) - ISBN: 9780307379788