Like others have said, we live in a society that simply does value and promote the relevance of relationships and the importance of keeping them together. We live in a ultra-indulgent, hedonistic, me-first, me-second, as long as I am happy environment that only encourages loyalty and commitment as long as the relationship is rosy.
Not to mention, our media does a great job of pushing situationships to the fore. Maybe the most popular genre in media today, reality tv, does a great job of pushing storylines that revolve around tenuous dating situations that are simply sloppy and anything but loyal and committed. Characters like Stevie J and Peter Gunz have become the entertainment of the masses....and maybe even role models to a certain extent.
People can clown all day, but I would bet that a lot of dudes wouldn't mind being in that position or even give those dudes props on a certain level.
What's worse is that the women in these situationships are somehow promoted as being "strong" and "independent" for being able to deal with these kind of men. In some ways, they're even made out to be heroines. This, maybe unintentionally, makes this lifestyle seem less tragic than what it actually is and almost justifies its existence. And whether we know its entertainment or not, it gives credence to the many situationships that happen in real life. And now, those people can say "well at least I'm not alone."
There's many factors that cause relationships to be as fickle as they are now. Truth is, its not just women...men are plenty fickle as well. However, comparatively speaking, women in 2013 are much more fickle than in years past.
Peace