Montag has many qualities that make him receptive to Clarisse's influence in Ray Bradbury's Fahrenheit 451. His curious nature allows him to both listen to and reflect upon what she says to him. His dissatisfaction with his life also plays a key role. Lastly, it is evident within the first few pages of the book that Montag longs for someone with whom he can hold a meaningful conversation.
Before meeting Clarisse for the first time, Montag gets a feeling of anticipation, almost as if something unknown is just around the corner:
The last few nights he had had the most uncertain feelings about the sidewalk just around the corner here, moving in the starlight toward his house. He had felt that a moment prior to his making the turn, someone had been there. The air seemed charged with a special calm as if someone had waited there, quietly, and only a moment before he came, simply turned to a shadow and let him through.
Though unknown to Montag at the time, this passage symbolizes his desire for a stronger, more meaningful relationship with another human being. His marriage is strained, consisting of conversations about nothing with a wife who seems to lack the ability to think critically. His curiosity of Clarisse and her behavior makes sense once the reader witnesses the faulty communication between he and his wife:
"I wanted to talk to you. He paused. "You took all the pills in your bottle last night."
"Oh, I wouldn't do that," she said, surprised.
"The bottle was empty."
"I wouldn't do a thing like that. Why would I do a thing like that?" she said.
"Maybe you took two pills and forgot and took two more, and forgot again and took two more, and were so dopey you kept right on until you had thirty or forty of them in you."
"Heck," she said, "what would I want to go and do a silly thing like that for?"
"I don't know," he said.
She was quite obviously waiting for him to go. "I didn't do that," she said. "Never in a billion years."
Mildred clearly wishes to avoid talking about her recent suicide attempt, refusing to acknowledge that she even took the pills. Montag offers a forgivable excuse for what happened, but she continues to argue that she "didn't do" it. After giving up on this topic of conversation, he then begins to ask her about the show she is watching. Her response - that she has been given a script which entails her blindly agreeing with the other 'actors' - further highlights her inability to think critically.
Thus, when Clarisse enters Montag's life, he finally finds a kindred spirit. Though she occasionally gets on his nerves, it does not seem to bother him too much since he has this connection with her. Because of this, it does not take long for her influence over him to take effect. After admitting to him that she sometimes catches raindrops in her mouth - something he has never done before - she leaves him to ponder this encounter:
And she ran off and left him standing there in the rain. Only after a long time did he move.
And then, very slowly, as he walked, he tilted his head back in the rain, for just a few moments, and opened his mouth...
Overall, Montag's curiosity, unhappiness, lack of human connection, and desire for meaningful conversation are the qualities that make him most receptive to Clarisse's influence. She represents a version of him that has lain dormant since he was a child. Unlike his wife, she incites feeling in him; even when that feeling is negative (such as when he finds her "peculiar" or "aggravating"), he can look past it toward the next meaningful topic of conversation.