Think back to the first time the chick you're with did that annoying thing she does. It was no big deal, you laughed it off.
Then compare that with when she did it again, last week, for the hundredth time.
Now imagine shes been doing that thing for 50 fucking years, she doesnt even pretend not to do that annoying thing anymore, and gets annoyed at you for even noticing it.
I'm kind of surprised there arent more old spouse murders, and am always sincerely impressed with the multi decade marriages.