There are bitter divorces
Friendships break up
Family estrangements
Business-related disconnects
I think that must be a really big shock unless the people involved never really loved each other and only pretended to for whatever reason.
I think it's more likely that married people will stay married when they dislike each other if they have young kids or if a divorce is too expensive. So they stay married and live separate lives and hopefully don't kill each other.
Friendships that were once very good solid stable close can change and that is always sad
Family estrangements occur all the time for whatever reason
A job you once loved becomes dreadful and terrible and you leave
It's all part of being a human being.