Some of the anonymized acecdotes I've heard second hand with regards to why the well-adjusted suburban upper-middle class gets divorced is that it's often due to the wife getting paranoid about getting ace'd out of the financial picture in favor of the children in her old age. Another reason is because the old man gets utterly infatuated with a younger lady and he throws his brain out the window. Alcoholism can also be a factor.

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.

For the same reason dating couples breakup: because something changes, externally or internally, that makes staying together less desirable than splitting up. I've been married for 3+ years, and been with my spouse for almost 10. We have a child. There is, to put it lightly, momentum that keeps us together along with societal pressures (a dad married to the mom is superior to the dad divorced from the mom, all other things equal.)

I'm not posting to comment on whether this pressure is good or bad, but merely to point out it exists. We live in a culture that celebrates weddings and proposals and scandalizes divorce (look at any tabloid for copious examples), but has little to say about the tenure between.

The trend of dating long term, co-habitation, and finally "tying the knot" late in the relationship is an interestingly recent development in America. I'm curious to see how the data on US divorce evolves as this trend matures. Most of my peers (I'm late Gen X) and Millenials I know are following this pattern.

I suspect it is a direct reaction to generations raised in divorced households, but I have no proof of this.

EDIT: small clarification.