This is a good article that outlines the reasons that relationships end. For me, trust is a HUGE reason;
"Most people don't go into a relationship anticipating the end. Instead there are likely feelings such as fluttering butterflies in the stomach, happy thoughts in the mind, and anticipation in the heart. Emotions are all about the excitement in getting to know this new person better as the seeds for the relationship are planted.
Sometimes these seeds flourish and grow into sturdy and hearty plants, other times they wither and don't get far beyond the seedlings and budding stage. Then there are those special relationships which become firmly entrenched and grow strong roots.
Whether a relationship is long-term, short-lived or a seemingly permanent commitment, it is hard to say from the beginning what the future holds for any relationship. Despite high hopes and hard work, sometimes even the best of the deepest connected relationships can sadly come to an end due to bad judgment, circumstances or other unexpected change.
The reasons for the ended relationship may vary, but for most people many of the same kinds of feelings exist when a relationship ends. The most common feelings associated with relationships ending are sadness, despair, longing, anger or fear.
With so many negative feelings linked to breakups, the question of why relationships die begs asking. Why would anyone want to experience the heart wrenching emotions that accompany breaking up with a significant other? Why do relationships, regardless of length, die?
There could be several reasons why this occurs. When a relationship runs its course, these are some of the most common reasons why either one or both partners come to the realization it is time to end the partnership:
*Dishonesty
Lies, big or small, can permanently harm a relationship. Trust is the primary building block to any relationship and once this piece of the foundation is cracked, often the crack widens until the foundation crumbles completely.
While it is true many relationships can survive white lies, and even sometimes larger untruths, dishonesty of one partner to the other often causes irrevocable damage...."
http://www.helium.com/items/1469025-why-relationships-sometimes-come-to-an-end-causes
Source: Helium.com