This could be a number of reasons, and all depends on the circumstances.....

1. They may have found someone else to love...

2. They may have fell in love too quickly and realised that the person they thought they loved had changed maybe...or

3. Maybe they think the grass is greener on the otherside...I know that is an old saying but sometimes I think people feel trapped in a stable relationship, they are very restricted to do what they please when they please.......

At this day and age we are certainly living like days of lives..... I hope this helps.