Why do long-term couples suddenly break up after many years together?

I recently found out that two couples I used to look up to broke up after +10 years together. Both couples seemed to be made for each other, lived together for many years, no financial issues and I always thought they would eventually end up getting married and have kids. What other issues could've caused the break up?

Why do long-term couples suddenly break up after many years together?
Post Opinion