If you look at the opening words, it says, "It was a pleasure to burn." Bradbury goes on to describe the sheer delight Montag gets from burning: "Montag grinned the fierce grin of all men singed and driven back by flame." He derives great pleasure from burning, and even in his sleep "the fiery smile still gripped...his face muscles." Later however, he quakes to burn Mrs. Blake's house. Montag feels the woman's "quietness a condemnation" instead of mocking it as he might have done before. Montag is so afffected by this that he stays home "sick" the next day, and in essence, is never really the same again.
Reasons for this change? 1. Clarisse McClellan asking, "Are you happy?". He realizes he is NOT happy, not even close, so sets about figuring out why. 2. Mildred's suicide only emphasizes his unhappiness because he realizes that no one is happy at all; he wonders how many more times she will try to kill herself, and why she is so disturbed. 3. Clarisse and the dandelion trick about being in love. He tries defending it, "I am, very much in love!" but goes home and realizes the empty shell his relationship with Mildred has become. These events get him thinking about what went wrong. Mrs. Blake's house is one of the final things that pushes him over the edge, to Faber, to reading books, to rebellion and redemption.