Character Analysis
Although Peter frequently criticizes Mary for being "sheltered from realities" and "living in a sentimental dream world," we think he comes down a little too hard on her (5.61). First off, we'd argue that she's more aware of reality than she lets on. Secondly, his critiques conveniently miss out on the fact that he needs her to help him escape from reality, too.
Throughout the novel, we see each character fall victim to some sort of delusional "dream," Peter included. Is it rational for a scientist like John to obsess over a sports car? Or for Moira to obsess over Dwight? Or for Dwight to pretend that his family's still alive? As Moira so eloquently states: "None of us really believe it's ever going to happen—not to us. [...] Everybody's crazy on that point, one way or another" (4.41).
As for Mary, her "dream" is her garden. The garden also happens to be Peter's dream—think back to when "they went on happily planning their garden for the next ten years" while Moira and Dwight were at church (4.36). Although Mary might be more gung-ho than Peter, he gets just as much out of the experience as she does.
To our surprise, Mary reveals another aspect to her personality in her final moments. No longer is she a fanciful young lady dreaming of garden seats—now she's the woman who tells Peter to "get this all over" with and who refuses to look at their daughter because she'd "rather think of her like she was" (9.226). Although Mary might seem to buy into her fantasies wholeheartedly, we think she understands their purpose (i.e., as a coping mechanism) better than she lets on.