I think in different stages of relationships people are anxious about Overcommitment, Acceptance at that Level, and Future Orientation.
-Friend
-Attraction
-Extra-Friendly Friend
-Pseudo Relationship
-Introduction of Relationship
-New Relationship Energy Phase
-Comfortable Relationship
-Optimistic Relationship
-Pre Engagement Relationship
-Engagement
-Post Engagement Planning Phase
-Pre Wedding
-Marriage
*For this Phase Exclusivity is not an expectation but, attention and flirting are normal and accepted.
*For this Phase Attention and Affection is Exclusive and it is the beginning of officially being in a relationship.
*For this Phase Both Parties are Open Trusting and eventually become Understood Fixtures in each other's Lives.
*At this point both Parties are looking Forward and Planning out the rest of their lives together.
For Different stages people have Expectations in accordance with the direction they see the relationship heading.
Different stages also come with rites of Passage, like meeting parents, opening diaries, explaining pasts, leaving toothbrushes, Facebook statuses etc.
Other reason's I've left:
-They were Blonde
-They love horses
-They took themselves too seriously
-Country Music
-BurningMan
-They had our life planned out before we said the "L" word.
-They had no way of expressing themselves.
-Vegetarian
-Weed
-Alchoholic
-Irrational Fears (Like the dark, open windows, being touched)
Reason's They left Me
-Pushing for too much Commitment
-I broke their leg Jet Skiing
-I was not Attractive Enough
-Too much Distance
-Too Prideful or Arrogant
-Too Irrespective of the Law
-Too Adventurous (Reckless)
I apologize if this is too indepth.