In Ray Bradbury's Fahrenheit 451, the Mechanical Hound is only one thing that the author uses to caution us against dependence upon technology.
The cars travel so fast that people can no longer see things along the side of the road, such as cows or flowers. These things are simply blurs. Drivers may know what the blur represents, but they are distanced from the natural world by driving cars so fast.
Have you ever watched the jet cars racing on the boulevards down that way?...I sometimes think drivers don't know what grass is, or flowers, because they never see them slowly...If you showed a driver a green blur, Oh yes! he'd say, that's grass! A pink blur! That's a rose garden! White blurs are houses. Brown blurs are cows.
Pedestrians are also put at risk, as it is not usual that people on foot are often killed—in fact, Montag is almost killed while running away at the end of novel. The car that misses him backs up with the seeming intent of finishing what the driver had failed to accomplish in the first place. Clarisse notes that the government prefers speeding in cars, even though the results are often deadly:
My uncle drove slowly on a highway once. He drove forty miles an hour and they jailed him for two days.
Ironically, Clarisse is killed by a speeding car. The technology to drive so fast not only keeps drivers from noticing details of the world around them, but also takes lives—which society is disinterested is stopping.
The TV walls in the house also distract its occupants from paying too much attention to the world at large: the programs are meaningless, made up of mindless propaganda. Massive numbers of people spend the day mesmerized by these forms of technology, forgetting how to think independently or how to question what is happening around them.
The Seashell that Mildred (and countless others) uses each night do not lull the users to sleep, but feed their minds with ideas and images supported by society (which is slowly self-destructing). The user rests in a trance-like state.
His wife stretched on the bed, uncovered and cold, like a body displayed on the lid of a tomb, her eyes fixed to the ceiling by invisible threads of steel, immovable. And in her ears the little Seashells, the thimble radios tamped tight, and an electronic ocean of sound, of music and talk and music and talk coming in, coming in on the shore of her unsleeping mind.
It is when using the Seashells that Mildred attempts suicide—quite possibly the result of depression: though it is unclear if she does this purposely or only because she is so numbed to her environment, separated from self-awareness and the world around her.
When Mildred overdoses, Montag has to call medics that deal with overdoses at an alarming rate.
They had this machine. They had two machines, really. One of them slid down into your stomach like a black cobra down an echoing well looking for all the old water and the old time gathered there...
Montag hates the machine. And he hates the attitude of those who are running it, that stand casually smoking while the machine works. They are technicians, not doctors. They show no empathy: much like the machine itself. The importance of personal touch and compassion have been lost.
Like the other forms of technology, the Mechanical Hound is programmed to attack anyone that is a threat to the norms established by the government. While in another society such an animal might be of benefit to apprehend criminals, this machine has been programmed to attack those that do not conform to society's norms, people that society deems as dangerous.
Overall, it would appear that Bradbury is cautioning the reader against allowing technology to aestheticize people to the point that the human component is lost. In the novel, people have become apathetic, forgetting the importance of individual power—something that Montag's government would have society's members willingly believe and accept.