So some may be programmed with stay-with-mate timers. I'm not. I know many happy couples who have been together for 40+ years. And there's those sad cases of long relationships where one partner dies and the other dies soon after... the so-called broken heart syndrome, ie stress-induced cardiomyopathy or takotsubo cardiomyopathy.  

There's other kinds of timers also. Like the timer some women seem to have, that goes off the moment they deliver the couple's last child.

but each small or large event in our lives contributes to our behavior which is based on (often) sub-conscious motivations which can appear to all at once manifest themselves by deciding to leave the person we've been with for 30 years, or perhaps to decide that one no longer likes the taste of tuna fish.  It isn't just for big life events, after all.

But like an earthquake, there is a lot of build-up below the surface that goes undetected before the big moment strikes

Maybe guys have an equivalent. I read somewhere that we lose our brain's happy cells and get grumpy.

-- Modified on 7/12/2015 1:59:38 PM

The entire concept of monogamous marriage being a normal state is flawed.  It is not how human beings evolved.  It is an artificial construct of our post agrarian society. Read "Sex at Dawn; the Prehistoric Origins of Modern Sexuality"  for a full and lively discussion of this concept. Anyone who survives 20, 30 or 40 years of monogamy deserves a prize.  But ultimately, when you see your life passing by and your needs being unmet, you just can't take it any longer.

When we got married wife stated she "could take it or leave it"  sex that is.  She was untruthful.  She really didn't want to & avoided sex as much as possible.  So it was once a year, if I got lucky.  Then it went to zero.  I decided I wasn't ready to be celibate...  my only choice was to get it someplace else.  
I think the correct description is asexual.