It's certainly different for everyone, no two couples are exactly alike.
For some, they eventually get familiar and start to take each other for granted -- no longer making the effort to express love and attraction towards each others.
For some, perhaps their idea of love was rooted in issues of co-dependency or fear of being alone rather than True Love -- and it just wasn't enough to keep the bond together.
For some, it could have been pressure from family to get married, but in the end there might never have been love...perhaps not even attraction.
Who Knows! -- At the end of the day, I think it's important to love and value the people in our lives (even our Ex'es) for contributing to our life experience, no matter how brief or long we shared our time together.
Lotsa Love, ~Tiki