By using my and some of my friends experiences of life, I've come up with the following theory why a relation ends. A relation is seriously at risk when one of the parties has met a new possible partner that is perceived as "much better" than the existing. That's it. The possible partner doesn't have to be a real option, but can still kill an ongoing relation provided there is a strong unfulfilled need. We all have needs, but if there is some important need that we don't get fulfilled for a long time in a relation, it is just be a matter of time before the relation is over.
Our needs tend to change somewhat over time as we grow older, which is probably one of the reasons why so many couples break up. Another reason why there are so many singles could be an increased selfishness and an inability to see other peoples' needs.
What do I know? I'm no expert...
