Many long-distance relationships are the result of not having a better answer to shifting circumstances that require two people in a relationship to live in different cities or regions.
With lease arrangements, career concerns and indecision, long-distance relationships can represent a holding pattern. Life doesn't naturally maintain holding patterns for very long. A couple in a long-distance relationship may not even notice the seismic change that is pulling them apart emotionally. Interests, values and friends may cause them to drift apart slowly and subtly. Or, depending on how different the two living environments are, these foundation-shifting changes may occur quickly and noticeably.
Not all relationships are built to last, and the ending of a long-distance relationship can only sometimes be blamed on the distance alone. There's always the possibility the distance served as a catalyst for something inevitable. Some relationships simply aren't workable in the first place, and attempting to maintain them at a great distance can only serve to illustrate it with additional clarity.
Unpredictable X-factors occur: One day, someone wakes up feeling like he or she isn't in love anymore, or meets a new romantic interest. Careers advance and new opportunities present themselves that only serve to move people even further apart on the map than they already are.
Many long-distance relationships end because the world keeps turning, but fortunately for those whose long-distance relationship ends, life goes on for them, as well.