2 reasons for this:
I) More than meets the eye
II) She grew tired of this relationship.
When someone is tired of this relationship (like someone else, or
simply tired, etc), and when their other partner is basically a
good bf/gf, there is no reason to dump them because they never did
anything (No two timing, no possessive, etc). In the end? the
excuses they receive would sound really quite lame. 
There is certainly more than meets the eye. Either she never tell
you or you never post it. Well, some question i could like to ask
is about the same as Uncle Devil and one more thing: Why did she
wanted to break you the previous time? Reasons being? Situation
involving? etc, etc.
Cheers
