At this moment in the book, readers have just experienced a scene change from Montag having a talk with Clarisse which made him more aware of and happy about the world around him. Then the scene shifts to Montag at work, which could have been just a few moments later. Before Montag greets the dog, Bradbury narrates:
He went out to look at the city and the clouds had cleared away completely, and he lit a cigarette and came back to bend down and look at the Hound. It was like a great bee come home from some field where the honey is full of poison wildness, of insanity and nightmare, its body crammed with that over-rich nectar and now it was sleeping the evil out of itself.
This is a parodoxical statement because it alludes to the power of the venom within the Mechanical Hound, but then make the assumption that the evil can acutally be extracted from the Hound with sleep.
The turning point of this entire situation for Montag and his development come with the idea that he actually demonstrated human emotion and thought. He considered the dog's abilities, and he actually thought kindly of the dog for a moment. This is Clarisse's influence.
Ironically, the dog is able to read human thought and recognize change. The dog snapping at Montag is the evidence that Montag is open to being enlightened about what is wrong with his dystopian society.
Hope that helps.
Montag feels controlled by the government now