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I'm not being facetious, but isn't this exactly what some promise or vow at the time of entering into the marriage?
Isn't that a reason? Because you promised to "stick it out", for better or for worse?
Not sure of the purpose of his promise otherwise.
Perhaps the point is that this promise/vow should be removed from "traditional" marriage ceremonies (and by traditional, I mean where the couple doesn't write their own vows), as it may be meaningless. Better that one promises to stick it out so long as one remains happy and fulfilled. (Again, not meant as a snark; meant seriously.)