The most common reason is compatibility, or the lack of it. People don't really DATE anymore, they just meet someone, feel some attraction and butterflies, and BOOM, they're in a relationship. It might be months or years before they actually start talking about what their goals and dreams and values are, only to find out they have HUGE differences. The gaps are often too big to fill. By dating for a period before they let themselves get too attached, and by talking a lot during that dating time about where they've been, where they want to go in life, and how they plan to get there, they can discover deal-breakers before they're committed, and realize that it isn't going to work and go their separate ways BEFORE they've been together for months or years. Problems can be worked out in advance, and thus avoided in the future.
Another problem is that a lot of people weren't taught how to handle life's problems and challenges, and so when things get tough, as they eventually do for everyone, they don't handle that in a healthy way. They self-destruct, or they turn to other things to help them avoid their problems (drugs, booze, cheating, spending, avoiding, etc.). Those kinds of things will tear even a previously healthy relationship apart.
There are plenty of other reasons too:
- financial stress
- failure to grow and adapt to change
- mental/emotional/health problems
- unrealistic expectations (naivety/immaturity)
If you start with a solid foundation, with a person you've vetted and are reasonably compatible with (no big red flags), and you both work to keep your relationship healthy and learn to communicate with each other always, chances are, you'll be able to survive anything that comes. So many people just don't do that, though.