Why did julia jump out the window?

As Robbie and Julia begin to spend more time together, they start to become closer. Julia asks Robbie what went wrong with Linda, and Robbie admits that he should have known that Linda wasn't the one, because once, when they were flying over the Grand Canyon, Linda wouldn't change places with him to let him look out the window, even though she had seen it before. Robbie asks how she knew that Glenn was the one, and Julia says that she knew she wanted someone she could grow old with -- "and Glenn will make a really good-looking older man."