Why do we fall out of love?

When we fall in love with someone our whole world seems somehow brighter, and as the songs say, the birds sing sweeter and the everything in the garden is rosy. We get all goose bumpy and can't wait to see that special someone, and really hate it when we are apart. We are somehow glued to each other, and it all seems wonderful and you want it to last forever. You hang on each others words, and have perhaps frequent and fulfilling sex. All of the above are seen as completely natural and so sought after that practically everyone wants this romantic and beautiful experience. If this is what love is supposed to be then why doesn't it last?, and when all these strong emotions have run their course what is left.? Some say that once romantic love subsides then true love really kicks in, and in a sense this is true. But why is it then that the divorce rate in most western countries has been rising steadily year on year?. Also if people were honest many of the marriages that do last are in a state of open or secret hostility, that stumble from one arguement to another. The man is on the golf course at weekends, or down at the pub, anywhere than were he should be. The woman resents this, and usually takes her frustrations out on the kids, or she like her man, leaves the kids with her mother, and takes herself off so spend time with her friends ,and they all swap stories about their selfish husbands. If they do spend quality time with the family many tensions rise, because mostly it is done out of duty and not real love. Couples that are at war with each other, are at war with themselves as well. All of this tension and secret hostility effects the children, and sets them on the path for their own troubled adult lives. Surley a happy marriage would mean that when not earning the essential income for the family, that the couple would want to spend every available minute with their loved ones? Why is it then, that romantic love doesn't last?