When Montag arrives at work the first day, he approaches the Mechanical Hound which growls and extends its silver needle toward him. It keeps extending it and retracting it, as if not sure if it should attack. When Montag steps back, the Hound steps forward, obviously becoming the aggressor. Montag complains that the Hound doesn't like him and hints that maybe someone manipulated its calculators. Its calculators were set to amino acids, sulfur, butter fat, and alkaline. All of the firemen's data were set up in the Hound's memory so that it would not attack the firemen.
The problem is that Montag had already started collecting books, and Clarisse had started Montag's process of "thinking" and "feeling" Many believe that the process of thinking and definitely "feeling" create a chemical and electronic reaction. (see website below). According to Wikipedia,"A chemical reaction is a process that leads to the transformation of one set of chemical substances to another". If Montag had a chemical reaction because he was starting to think and rebel, then the Hound would have recognized it. It wasn't fully developed yet, so the Hound wasn't sure. If it had been sure, Montag would have been dead.