I always thought it came down to one of two reasons (or both).
- Money and the lack of properly managing it.
- Diminishing interest as a relationship matures as well as a lack of open communication that's needed to keep a healthy relationship. Part of that is also due to not communicating well enough before getting married so one knows exactly who they are marrying ("love is blind"). That generally leads to one spouse to eventually cheat on the other or passively give up on the relationship.
- Money and the lack of properly managing it.
- Diminishing interest as a relationship matures as well as a lack of open communication that's needed to keep a healthy relationship. Part of that is also due to not communicating well enough before getting married so one knows exactly who they are marrying ("love is blind"). That generally leads to one spouse to eventually cheat on the other or passively give up on the relationship.