Obviously somewhere in history someone made the observation that humans are not monogamous and implemented the rule to get married in front of tons of witnesses and promise our undying love to someone, so we can be held accountable if we dare break the collective illusion.
That probably happened somewhere around the transition from tribal to city culture, when money was introduced and the economic need for such a relationship was born.
In my own experience, the "love" feeling (hormonal rush) lasts for about 3-4 yrs, before my head starts turning otherwhere. That coincides with some scientific assumptions of life in tribal times, where a couple would stay together for roughly 4 years to raise a child, and the whole community would raise it after it was weened off the breast and learned some basics. The mother and father would split, and have other children with other partners, thus ensuring the much needed diversity for the DNA to stay healthy and adaptable.
Somewhere in history we started going against our biological nature and got ourselves bound into lifelong monogamy with illusions of love that were supported by fairy tales since we were little... by the time we felt our first hormonal rush, we were completely brainwashed and believed this sensation to be "love", and that it will last forever. By the time we realised that it is not so, we were already married and fucked for life. Why? Because we had promised each other eternal undying love in front of tons of witnesses.
Fucked up, eh?
Get out of the illusion into real Life http://desteniiprocess.com/
Support the solution to the fuckup of love http://equalmoney.org/
Bring yourself home http://desteni.co.za/