Throughout the novel, the reader can sense that Guy Montag will change and transform society from what it was. His own wife, Mildred, turns in Guy Montag. Montag has a collection of books, which are illegal in this society. As a fireman, he stole books as he burned houses. Mildred turns in Montag because she only cares about the "family", which is a TV parlor. Mildred calls the fire alarm to turn in Montag and leaves Guy. Montag's boss, Beatty reveals that he was just toying with Montag at the fire station, as Mildred's friends had turned in an alarm even before Mildred did (and before Montag reached the station). Mildred values nothing in life as highly as her three TV walls. She does not understand her husband and really has no desire to do so. This is why she turns him in.
Mildred is Montag's only family (I guess).
Montag's reaction was betrayal. Mildred's reaction is 'like she doesn't care about him.' Montag, instead of feeling mad feels pity and regret. In the book, it doesn't show any other family other than Mildred and himself, but Montag probably felt surprised to find himself at his house to burn it. The author's purpose in the novel was to get Montag to step in action from hiding books to help society towards the end.