How about this for a theory, most marriages will go through periods of ambivalence, where either or both spouse could take or leave it, staleness, boredom etc etc, it might feel like it'll be that way forever, it might actually be that way forever. But my suspicion is that not enough people make the effort to try to get over that hump, they throw in the towel too quick, or that for selfish reasons look beyond the marriage for a bit of excitement, as stoicism is not something which is prized in todays society (though it might be for the overall good of the family).
They probably could go through the motions if they wanted (pretend they love their spouse - dont forget most of us will do this from time to time as we're not sure our heart is in it), but they dont bother.