- 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.
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.
The primary reason for anti-walkability is to keep "riff-raff" out of the nice suburbs. The white flight from the cities in the 60s and 70s was a huge event in American urban planning and those whites who wound up in the suburbs don't want any of that inner-city stuff they got away from downtown from mucking up their suburb. It's horrible but it's the truth. It's one of those awful little secrets of the polite suburbs, like why people get divorced, that everybody knows about but nobody wants to talk about.