Before meeting Clarisse and Mildred's possible suicide attempt, Montag has no reason to think that their marriage isn't going well. However, his slow movement towards individual thought begins to drive a powerful wedge between himself and Mildred, as he reads books and realizes how banal the television programming truly is:
"It's sure fun," she said.
"What's the play about?"
"I just told you. There are these people named Bob and Ruth and Helen."
(Bradbury, Fahrenheit 451, Google Books)
Prior to meeting Clarisse, who talks about how people just repeat things that they heard on TV or that they heard other people say, Montag would never think to question the actual "about" of the play; the question is completely alien to Mildred, who is content to live inside the TV walls without caring what they actually say. Later, as Montag reads and collects books, their marriage becomes more and more antagonistic, culminating in a (culturally shocking) scene where he reads to their guests.