The Bark Bites Back at 2012-02-28 12:28:21:
Ahhhh! The "you are a parenthesis"! That's it! She tells him "you are an escape. You are a break from our normal lives".
What she may not really understand and he subsequently comes to realize is, he loved his normal life... until he met her. Now he wants something more.
It's a great example of the grass is always greener. He is her escape from her family, her real life, and she provides an escape from his life of solitude - only given a taste for it, he wants something more.
Just another example of contrasts between the characters that I didn't think too much of beforehand whereas the dichotomy between Ryan and Natalie is readily apparent.
Both of those relationships provide contrasts in different ways and make up for the heart of the movie itself, the emotional "throughlines".