[Symbol]

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.