Well, when I married my now ex-wife after 12 years together and 3 kids I thought that was it and we would be together forever.
About 3 years later we were hardly talking, carried on for another 2 hoping things would get back to where they were ...but no luck. We split.
Circumstances change, can't explain it, they just do.
I even predicted the final split ( and how it all panned out ) on this very forum, on one of the old 'what are you doing this weekend' threads