Of most relationships maybe, but not of most marriage type relations though. People marry and divorse after the 5th and 10th and 15th etc. year all the time...
That's a sad reality. If my father hadn't died 16 years ago, my mother would have divorced him.
People change. Sometimes a lot. You've fallen in love with a certain person and one morning, decades later, you suddenly realize this person is no longer with you.
