In Ray Bradbury's Fahrenheit 451, why does Montag becomes angry with Mildred?

There's a passage early in Ray Bradbury's classic depiction of a futuristic dystopian society, Fahrenheit 451, in which the author, emphasizing the passionless nature of his protagonist's existence, save for joys of executing his mission of burning books, depicts Montag's arrival at his home at the end of his shift only to encounter the familiar sight of his wife, Mildred, tuned out of reality:

"Without turning on the light he imagined how this room would look. 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. The room was indeed empty. Every night the waves came in and bore her off on their great tides of sound, floating her, wide-eyed, toward morning. There had been no night in the last two years that Mildred had not swum that sea, had not gladly gone down in it for the third time."

With this introduction to Montag's home life, it is unsurprising that, once this repentant fireman has evolved into an opponent of everything in which he had earlier believed, he grows increasingly angry towards his wife. As Montag, having witnessed the woman deliberately burn herself to death in the fire that consumes her books, and having begun to question the nature of his existence as one of those tasked with carrying out the burning, he grows more and more intolerant of Mildred's almost robotic dedication to living under the government's rigid strictures. Mildred, as depicted in Bradbury's novel, is the consummate good citizen. 

In Fahrenheit 451, Bradbury has conjured up a fictional society in which an autocratic regime denies the populace under its control access to all sources of information and knowledge. Books, as Professor Faber and, most significantly, Captain Beatty enlighten Montag, contain knowledge -- knowledge that could raise inconvenient questions in the minds of the public with potentially unpleasant ramifications for the government. A pliable populace is essential to the regime's ability to survive with unquestioned authority. Mildred is the very embodiment of what the regime wants of all of its citizens. Her's is an emotionless, empty existence. She stay's permanently tuned-out of the world around her, evident in the following passage describing Guy and Mildred's typical morning:

"Toast popped out of the silver toaster, was seized by a spidery metal hand that drenched it with melted butter. Mildred watched the toast delivered to her plate. She had both ears plugged with electronic bees that were humming the hour away. She looked up suddenly, saw him, and nodded. "You all right?" he asked. She was an expert at lip-reading from ten years of apprenticeship at Seashell ear-thimbles. She nodded again. She set the toaster clicking away at another piece of bread."

So brainwashed is his spouse, that Montag is compelled to attempt to conceal even from her his possession of the book he has secreted away. That Mildred does discover his secret, and uses it against him, including holding her husband up to ridicule before her friends:

"Guy's surprise tonight is to read you one sample to show how mixed-up things were, so none of us will ever have to bother our little old heads about that junk again, isn't that right, darling?'

"He crushed the book in his fists. "Say `yes.' His mouth moved like Faber's.

"Yes."

Mildred snatched the book with a laugh. "Here! Read this one. No, I take it back. Here's that real funny one you read out loud today. Ladies, you won't understand a word. It goes umpty-tumptyump. Go ahead, Guy, that page, dear."

Montag becomes angry with his wife before Bradbury's story even begins. Their marriage has clearly been little more than a formality for some time, as the early reference to Mildred lying in her own bed suggests. His break with the regime he has loyally served, however, expands the chasm between husband and wife astronomically. A once passionless relationship has evolved into one of hostile antagonism. His growing disdain for the system he once served cannot coexist with a wife who is the embodiment of that system.