As the novel starts, there are two major forces of change that affect Montag deeply. The first is Clarisse, the young girl he meets and befriends; she is capable of individual thinking and asks him questions that make him uncomfortable because he cannot rationalize them. Her influence remains in him long after her probable death; his own curiosity is entirely spurred from her questioning and outlook on life. The second is Mildred's possible suicide attempt:
There are too many of us, he thought. There are billions of us and that's too many. Nobody knows anyone. Strangers come and violate you. Strangers come and cut your heart out. Strangers come and take your blood. Good God, who were those men? I never saw them before in my life!
(Bradbury, Fahrenheit 451, Google Books)
Although Mildred may have deliberately taken all her sleeping pills, she claims to have no memory of the act, and does not repeat it. However, Montag suspects that her mind is so fractured and superficial from her constant diet of meaningless TV that she simply forgot that she'd already taken her nightly dose. Returning to the bottle again and again, she simply had no memory of the automatic action, as it was something that took place between watching TV, and so had no meaning for her.