My wife and I (in our 50’s) have noticed how many couples we know are pretty unhappy with each other. It’s surprising how bad they’ve let their relationships get.
Marriage is a skill and not a feeling.
Not all of us know how to nurture a marriage to keep it alive
I think it can happen if the two are so wrapped up in work and kids that ‘they’ i.e. the couple don’t exist anymore.
then, when they retire and/or the nest empties they are now two strangers staring at each other