I think a high percentage of marriages go through a challenge/change at or around the 10 year mark. I think this has to do with stages of personal growth that is inevitable for each person.
Life changes each of us and no one really means to out grow a partner but it happens a lot. I think that nature plays a big role in this because nature’s main drive is about reproduction. This does change as a person gets older. There is a chemical aspect that is beyond ones awareness.
I think there is an ideal and that doesn’t always go along with how we go through changes.