The following post on Yahoo! Shine is worth a read. The author explains that the most common reason couples split up is because of core values clashing. Some of these values include religion, political opinions, family values, and work ethic. The article explains that these differences may be overlooked in the start of a relationship but once the relationship becomes long term they may cause a breakup.
Definitely worth your every attention:
The Most Common Reason Couples Breakup