"Why do couples break up?" is a cause essay, and would make an interesting paper. However, the essay may be more interesting if you narrow it a bit more. Here are some suggestions:
1. Why do high school couples break up?
2. What are the most important reasons college-aged couples decide to stop dating?
3. What are the reasons most married couples break up?
4. What is the reason people dating long-distance break-up?
5. What causes women to break up with a man?
6. What causes a man to break up with a woman?