The distance relationship between them. The "marriage" they have.
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Montag and Mildred have grown apart. They have nothing in common, especially when she gets involved with the "family" and he begins to consider how important books can be and are. She leaves him behind when she discovers his obsession about books and cannot be involved with him. She even tries to commit suicide because of the emptiness of their life and the way in which they have grown apart.