...and they're particularly likely to change under stress, like perhaps the stress of major lifestyle changes associated with raising kids. Child-related constraints on schedules, travel, and finances often stress relationships that were fine for years before. When those kids become more independent and able to advocate for themselves - which is a good and healthy thing - you have the additional stress of a decision process that used to involve two people involving three or more instead. It's just a different world compared to the couple-without-children state. For some it's a breaking point.
...and they're particularly likely to change under stress, like perhaps the stress of major lifestyle changes associated with raising kids. Child-related constraints on schedules, travel, and finances often stress relationships that were fine for years before. When those kids become more independent and able to advocate for themselves - which is a good and healthy thing - you have the additional stress of a decision process that used to involve two people involving three or more instead. It's just a different world compared to the couple-without-children state. For some it's a breaking point.