Mildred and Montag may have loved each other at one point, but now they live in a loveless marriage and basically just co-exist in their home. Mildred is obsessed with her soap opera family and watches television to escape her unhappy life. She overdoses on drugs to be able to bear her life or in a conscious or unconscious attempt to commit suicide.
After meeting Clarisse, Montag is disgusted with Mildred’s overdoses, and he begins to search for a better life. When he tells Mildred that he has books hidden in the house, she is unsupportive and terrified that they will get caught. Mildred eventually turns in Montag for having books to Beatty. We see the marriage crumbling when she runs out of the house and leaves Montag when Beatty confronts him and asks him to burn down his own home.
Montag has outgrown his relationship with Mildred as he is discovering the importance of knowledge and books. Mildred, unfortunately, has been so brainwashed by society that she and Montag would probably never be able to stay together. All love has been lost in their relationship as they grow apart physically and emotionally throughout the book. After learning about himself, Montag probably doesn't like the person Mildred has become in the novel.