The idea of a city, particularly somewhere like New York, is predicated on diversity - on heterogeneity. April Wheeler had a dream of her and Frank being different, of being "shining stars" in the world. The collapse of these dreams was not the result of her having kids or the family's lack of money, it was the existential crisis of homogeneity. Suburban living has the potential to create a situation that lacks the tension of difference between the people that surround us and the people with whom we come in contact. The Wheeler's moved to the suburbs "because of kids," but it is also because of their fear to "difference," which I use in reference not only to phrases like "dare to be different" but also in reference to the difference of non-likeminded people that you can't help but encounter on the subways and railways of major metropolises. Once they were in the suburbs, there was no chance of getting out because their imaginations and dreams faded until April eventually becomes an empty shell.
The crisis of homogeneity does not only happen in the suburbs, it can happen anywhere, but the ideas of defensible space (between the city, between people) and spatial privacy, and the proliferation of private transportation and the internet as public space, are all attempts to push tension out of our lives. Voting blocs get drawn very distinctly the more we territorialize into homogenous groups (ex. gated communities) and let the public sphere fail. On this blog, I've already pointed to suburban shopping malls and privately owned new-urbanist developments as examples of responses to the terror of urbanism, but suppose that all of this is also a result of a fear of difference.
The implication here is not that difference isn't sometimes painful or annoying (Jerry Seinfeld and Larry David have proven that it is a comedy goldmine), but maybe it is really necessary for the feeling of being alive for which the Wheelers were searching.