I don't think it's so much an issue of a "natural shelf life" as that both people need to continue to invest in the relationship to keep it from spoiling. Without consistent effort to fight the forces of entropy - changes, adversity, differing needs - all relationships are bound to go sour.

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Life is too short not to love and be loved....preferably multiple times in one night.

I think men talk to women so they can sleep with them and women sleep with men so they can talk to them. ~ Jay McInerney