It has been my experience that modern relationships are based around one night stands that become something more. Supposedly in the past they were based on other things, like prolonged security, perceived genetic compatibility, etc... It is my supposition that the increased globalization of society has created a perceived larger pool of potential mates, as well as easier access to them. Along with the large scale destruction of societal taboos on sexuality, the "always another fish in the sea" concept has gained more popularity. In this way, humanity has moved toward a quicker mode of determining mates (speed dating, slutting it up, etc...). The large scale proliferation of stories of metaphysical "true love" has also made people think that there is someone out there who matches their ideal of a perfect mate. As a society, this has led to looser bonds between people, as we look toward the horizon for the next best thing.