Alternative explanations for the evolution of monogamy and sibling cooperation
Monogamy evolved as a mating strategy: New research indicates that social monogamy evolved as a result of competition
Oxytocin leads to monogamy
Monogamy is an evolutionary trait of mammals. In humans, driving forces are largely genetic, including the production of oxytocin and phenylethylamine in response to pair-bonding signals & urges. However, the frontal lobes are by design capable of overriding certain instincts and impulses. The frontal lobes are also the primary centers of personality, and personality is of course formed by life experience.
I think the ability of the human mind to override instinct amalgamated with how more acceptable it has become culturally to divorce or be polyamorous, has led to a loosening of inhibition, and an acceptance of other impulses. The "serial monogamy" trend has much to do with the availability of partners, as well as the increasing cultural acceptance of it.