There could be a lot of causes. Maybe like Bob pointed out she might see marriage as a temporary thing. It seems cold hearted and cruel of her to leave and that is why I said maybe she has found someone else.
Maybe she is depressed and feels that leaving will make her feel better. People often blame other people for how they feel.
There has to be something causing her to want to leave. If she is depressed maybe she is blaming you for her feelings.
Maybe she has not found someone else, that was just a thought because I know so many people that this same thing has happened to and that was the reason.
