I think at least part of the reason is that the whole reason behind marriage in the US and Western Europe has changed. I was struck by this while watching an episode of the National Geographic Show "Taboo," which discussed cultures where the goals of marriage are very different. We in the West have this idea today that marriage is about Love. This is actually a very rare, relatively new idea. Marriages of the past were more about organizing society, dividing up the labor, and providing for future generations. In many cultures marriage is almost entirely an economic decision--not necessarily in the gold-digger sense, but in the sense that, in many traditional cultures, there is a certain amount of work that needs to be done, and if it's not done, there will be suffering and/or death.
For example, let's say we have a culture based around home gardening and livestock raising. In order to achieve adequate nutrition from the available resources, you must maintain a garden with X crops, you must care for the animals, you must maintain your shelter and the shelter of the animals, and you must protect your property from thieves and raiders. One person cannot do all of that by themselves. So in order to survive, you have to get married so you can share the work--you might take care of the animals and your spouse takes care of the garden. Kids make the work that much easier, and ensure that the civilization is preserved, so it makes sense for people to pair off in units that can produce children. This is not so much about love as it is survival--spouses are almost more like coworkers than life partners! Love might develop over time, but as far as society goes it is largely irrelevant--so long as your household works well together, the marriage is a success.
But today, we don't need spouses for survival. It is very easy to survive and have a fine life without getting married at all--in fact, it is almost easier to survive on your own than with someone else. You can rent or buy a suitable dwelling on one income, pay people to do any maintenance you are unable to do yourself, afford prepared food for yourself, and enjoy ample leisure time to do whatever you want. You can even raise kids alone without too much trouble, what with daycare and welfare and all the support networks that exist (being a single parent is no picnic, but it is very possible--in the old days it simply could not be done because of time and work requirements). The only reason we have left to get married is for Love and Happiness. And these are fickle things. They comes and go unpredictably, they rarely exist in a true form until many, many years together, and because love and happiness are all we are interested in, if we feel like we're not happy enough or we aren't in love enough, we simply leave and find someone else in hopes that they will make us happier and make us feel more love.
